Over a period of weeks this group of student radicals, which became known as the March 22nd Movement, held demonstrations and occupied various university buildings and when the police arrived to arrest them they were driven from the campus by other students who had just left their lectures. In response, the dean of the university closed the campus on May 2nd, and the student radicals moved their demonstrations to central Paris and the Sorbonne.
Around three-hundred students held a meeting on the 3rd May 1968 to protest the closure of Nanterre. This core of left-wing student activists were nervous, fearing an attack by a right-wing student group called Occident. To allay these fears some students guarded the entrances to the meeting armed with truncheons. For the university authorities this act of defiance was unacceptable and they decided to take action.
At around 5pm the police arrived. Representatives from the students negotiated a deal with the police for their dispersal. The students would lay down their weapons and leave the area. The women would depart first, and then the men, both in groups of twenty-five. In return there would be no arrests. Instead, after the women had left the police moved in, arrested the men and forced them into the back of waiting police vans.
The onlooking students, who had not attended the meeting, were enraged. They responded by shouting at the police. As the police attempted to leave, the swelling crowd blocked their path. The police responded with tear gas. More and more young people joined the crowd and eventually the a paving stone smashed through a window of one of the vans. This was to be the first of many. The fighting continued up and down the Boulevard St Michel until darkness fell.
Seventy-two policemen and innumerable students had received injuries. Around six hundred students were under arrest. The French student revolt of May 1968 had begun.
The events of May and June 1968 in France have long been the subject of debate. The Sens Public site has an essay by Chris Reynolds called May 68: A Contested History in which he analyses these debates.