Usually, there are about 20,000 or so in attendance at the Sunday Angelus in St. Peter’s Square.
Today, in answer to Cardinal Ruini’s invitation to turn out and show support for the Pope and his message, there were, it is estimated, about 200,000.
(Vatican Radio and other early reports – 200,000; Reuters – 100,000 – reality is probably somewhere in between.)
Various posts on the event:
(For those of you wondering….what is this Angelus? It is a relatively short event in which the Angelus is prayed and the Pope offers some brief remarks, usually on the Mass readings of the day. He does this from a window in the Apostolic Palace. (Here’s Katie and the Pope at the Angelus on 3/5, two years ago.)
Various reports. First Zadok, who was there:

It was a joyful moment of prayer with the Holy Father. The contrast with the scenes at La Sapienza couldn’t be greater. The police presence was very light despite the numbers present, whilst the protesters in the university had to be controlled by legions of riot-police. There was also a wonderful mix of people there of all ages, although it was surprising to note how many young people showed up. It was also interesting to see the banners of some of Italy’s trade unions present – not protesting, but showing that the Pope’s voice is welcome in the public arena.

(He has photos, too)
A blogging seminarian from the Diocese of Toledo, who was also there:

Perhaps the most outstanding thing about the whole address was what happened after the traditional, Buona Domenica a tutti, which usually concludes the day. To the resounding applause that continued to sweep the entire square, Benedict stayed in his window and kept waving (something he never usually does). In fact, he even went off-the-script and urged everyone to go forth into the world to deliver this same message of solidarity and peace in the face of opposition; you could tell, he was speaking from the heart, and what better response to a protest last week than a bolstering of fervency this week? In my mind, this is just another example of how God has brought something quite good out of a situation where divisiveness and ignorance

AsiaNews:

The pope greeted the crowds, but moved on immediately to his reflection, dedicated to the Week of Prayer for Christian Unity, which celebrates its 100th anniversary this year.
Benedict XVI recalled that “today the spiritual sons and daughters of Father Wattson, the Brothers and Sisters of the Atonement, are present in Saint Peter’s Square”. This is the religious community that began the tradition of the Week of Prayer for Christian Unity.
He then commented briefly on the theme chosen for the Week this year, “Pray without ceasing” (1 Thess. 5:17). “With this appeal, [Saint Paul] . . . wants to make it known that the new life in Christ and in the Holy Spirit brings the ability to rise above every form of egoism, to live together in peace and fraternal union, to bear each other’s burdens and sufferings willingly. We must never grow weary of praying for Christian unity!”.
The pope then invited everyone to participate in the solemn Vespers that he will lead on January 25 at the basilica of Saint Paul’s Outside the Walls, “to invoke from God the precious gift of reconciliation among all the baptised”.
It was only after the prayer of the Angelus that the pontiff turned to address the crowd that had gathered in Saint Peter’s Square “to express your solidarity” after the incident at La Sapienza. Amid the applause and the fluttering banners, Benedict XVI thanked everyone “from my heart”, and thanked Cardinal Ruini, “who promoted this encounter”.
Without argumentative tones, the pontiff recalled what had happened: “As you know,” he said, “I had very willingly accepted the courteous invitation extended to me to speak last Thursday at the inauguration of the academic year at ‘La Sapienza – Università di Roma’. I know this university very well; I respect it and I am fond of the students who attend it: every year, on various occasions, many of them come to meet me at the Vatican, together with their peers from the other universities. Unfortunately, as is well known, the atmosphere that was created made my presence at the ceremony inadvisable. I cancelled the visit unwillingly, but in any case I wanted to send the text that I had prepared for the occasion”.
And as he had done in the written speech that he sent to the university, he explained what the university should be in this way: “I am bound to the university environment, which was my world for many years, by the love for the search for truth, for debate, for the frank and respectful dialogue on the positions in question”.
The pontiff emphasised that this attitude is part of “the mission of the Church, which strives to follow faithfully Christ, the Teacher of life, truth, and love”.
Finally, the pontiff called upon all the people present, among whom were a large number of university students, to live the university as a “search for truth” and as “respect for the opinions of others”.  “As a professor ’emeritus’, so to speak, who have met so many students in my life, I encourage all, dear university students, to be always respectful of the opinions of others and to seek truth and goodness in a spirit of freedom and responsibility. To all and to each I renew my expression of gratitude, assuring you of my affection and prayers”.
After the greetings in the various languages, the crowd burst into applause and chants of “Viva il papa” and “Freedom”. The pope smiled and, waving goodbye, added: “Let us continue to live in this climate of fraternity, in the search for truth and freedom, in a shared commitment to a fraternal and tolerant society”.

More photos…
La Repubblica
Corriere della Serra
Yahoo News
Fr. Z has posted some video clips.
There was also a gathering in Milan – about 10,000 estimated to watch the Angelus:

Image Source
By the way, Communion and Liberation has been an important voice in this whole affair, gathering support for the Pope.
And an interesting Pope and academia tidbit, from Papa Ratzinger Forum – a translation of an article in Avennire (the paper of the Italian bishops’ conference)

Benedict XVI would be ‘ostracized’ at a Roman university founded by a Pope by a handful of professors and students who call themselves ‘free thinkers’.
While in the country that is emblematic of 1968 and its countercultural upheavals, at the Sorbonne’s faculty of philosophy, a young professor who is a non-believer, just dedicated a semester’s course work to the philosophical thought of Benedict XVI!
“This Pope is simultaneously revolutionary and very conservative,” says Jacob Schmutz, class of 1971, graduate of history and political sciences from the universities of Brussels, Cambridge and Paris. “He pursues a very daring interpretation, and quite symbolic, of the soul, defining it as something which can always be maintained by our existence.”
Schmutz is a maitre de conferences [academic aide in charge of arranging lectures] at the Sorbonne’s fourth unit, and in his course, he focused on the intellectual production of Joseph Ratzinger as professor in Muenster, Tuebingen and Regensburg in the 1970s and 1970s.
How did it happen that this lay intellectual – translator of Eric Voegelin, right-hand man of the philosopher Rudei Imbach (who is considered one of the major interpreters of Thomas Aquinas today) – managed to focus on Ratzinger?
“Through St. Bonaventure and St. Augustine, since I specialize in medieval philosophy,” Schmutz said to the French newspaper La Croix. At the end of a course on philosophy and the Middle Ages in 2006, he had given a lecture on John Paul II and Benedict XVI as students of the Medieval theologians.
The enthusiasm of the students led Schmutz to decide he would dedicate an entire semester – February to June 2007 – to the philosophy of the German Pope, from the ecclesiastical perspective, as well as the interpretation of Catholicism advanced by the author of Introduction to Christianity.
Schmutz belongs to the Pierre Abelard [of Heloise and Abelard fame] Center of the Sorbonne, and is the webmaster of the site Scholasticon dedicated to scholastic philosophy. Another [paradox, since the Center was hosted in the past by the Free University of Brussels, one of the sanctuaries of European anti-clericalism.
Schmutz does not hide his irritation for what he calls ‘the ignorance of the media’ and their usual ‘anti-Ratzinger vulgate’.
“Every time they report about him, it is to bring out the usual commonplaces about the cold intellectual and the opposition between faith and reason. But those are hardly his credentials.”
Rather, in Schmutz’s opinion, “Ratzinger is a true philospher. That is why he wins over students, even atheists.”
And yet, Schmutz’s academic interest is far from accommodating: “I find Ratzinger’s thought fascinating and ‘dangerous’. He believes that the only way to be rational is to be Christian.”
(editorial note – as Teresa at PRF notes, this is not accurate. But anyway..)
But despite that, he has dedicated study to Ratzinger’s thought with passion and intellectual rigor, without ostracism. With great respect for the then Prefect of the Congregation for the Doctrine of the Faith who, in 1999, was invited to give a lecture at the Sorbonne, and who chose to speak on “The truth in Christianity”, in which Ratzinger stated, “In Christianity, rationality became religion and no longer its adversary.”

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