But all the planning and organization counted for little in the face of the government's sweep arrests. More than 7,000 people were caught in the dragnet that first day. Never before or since have there been that many arrests in the United States on a single day. (Another 6,000 were arrested over three more days, most of them for blockading the Justice Department and the U.S. Capitol.) Many of the arrestees were ordinary people with no connection to the protest; they just happened to be where sweeps were taking place. Others were demonstrators who were arrested preemptively, without having committed any illegal acts. To transport the mass of prisoners, the police had to commandeer city buses; when even that wasn't enough, they hired Hertz and Avis rent-a-trucks.
The city jail quickly filled, even though the police crammed as many as twenty people into two-person cells. Another 1500 were packed into the jail's recreation yard. That still left thousands of prisoners, whom the police herded into an outdoor practice field next to RFK Stadium. Conditions were awful, with next to no sanitary facilities, blankets, or food. One anarchist wag made a sign proclaiming the football field, without much overstatement, "Smash the State Concentration Camp #1." The government had made a major misstep, which cost it public sympathy. People who had strongly disapproved of the Mayday Tribe's shutdown plan were appalled by the flagrant violation of civil liberties, and upset to see the nation's capital turned into an overt police state.
There was a class action lawsuit that came out of these illegal arrests, and I remember there was a fairly large financial award won on behalf of those arrested.
http://www.dailykos.com/story/2011/04/25/969961/-My-Meeting-AG-John-N-Mitchell-40-years-ago-this-week-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To show their disagreement with the war effort in Vietnam, thousands of protestors filled the streets of Washington to halt the day-to-day operations of the federal government. Between April 22 and May 6, 1971, the police arrested 14,517 persons, typically on charges of disorderly conduct. The government held more than 1,500 of these protestors at the Washington Coliseum. Of those charged, 871 proceeded to full trial on the merits. The Court of Appeals eventually ordered the District Court to enjoin prosecutions not supported by specific evidence—the vast majority of cases. The Court later ordered that the arrest records of thousands of protesters be expunged. In 1976, a class action lawsuit was filed on behalf of 1,200 arrested protestors against former Attorney General John Mitchell, former U.S. Capitol Police Chief James Powell, the District of Columbia, and others, alleging that the officials violated their First Amendment rights. In 1981, the parties settled.
http://www.dcchs.org/news/5_History.pdf