A mass dismissal of wrongful convictions ─ perhaps the biggest ever ─ unfolded in a Boston courthouse on Tuesday, as prosecutors in seven Massachusetts counties dropped more than 21,000 low-level drug cases tainted by the work of a rogue lab chemist.

The number of corrupted convictions was so unwieldy that the district attorneys used compact discs to deliver lists of cases they were dropping and those they believed they could successfully re-prosecute.

By day’s end, just a few hundred cases involving the disgraced chemist, Annie Dookhan, remained. The exact number may not be clear until Wednesday, when the court finishes processing the lists. The American Civil Liberties Union of Massachusetts, which fought for the dismissals, did its own tally and came up with 320 cases dropped from a total 21,907.