Dennis Kyne put up such a fight at a political protest last summer, the arresting officer recalled, it took four police officers to haul him down the steps of the New York Public Library and across Fifth Avenue.

"We picked him up and we carried him while he squirmed and screamed," the officer, Matthew Wohl, testified in December. "I had one of his legs because he was kicking and refusing to walk on his own."

Accused of inciting a riot and resisting arrest, Mr. Kyne was the first of the 1,806 people arrested in New York last summer during the Republican National Convention to take his case to a jury. But one day after Officer Wohl testified, and before the defense called a single witness, the prosecutor abruptly dropped all charges.

During a recess, the defense had brought new information to the prosecutor. A videotape shot by a documentary filmmaker showed Mr. Kyne agitated but plainly walking under his own power down the library steps, contradicting the vivid account of Officer Wohl, who was nowhere to be seen in the pictures. Nor was the officer seen taking part in the arrests of four other people at the library against whom he signed complaints.

A sprawling body of visual evidence, made possible by inexpensive, lightweight cameras in the hands of private citizens, volunteer observers and the police themselves, has shifted the debate over precisely what happened on the streets during the week of the convention.

For Mr. Kyne and 400 others arrested that week, video recordings provided evidence that they had not committed a crime or that the charges against them could not be proven, according to defense lawyers and prosecutors.

[...]

Seven months after the convention at Madison Square Garden, criminal charges have fallen against all but a handful of people arrested that week. Of the 1,670 cases that have run their full course, 91 percent ended with the charges dismissed or with a verdict of not guilty after trial. Many were dropped without any finding of wrongdoing, but also without any serious inquiry into the circumstances of the arrests, with the Manhattan district attorney's office agreeing that the cases should be "adjourned in contemplation of dismissal."