(CNN) Iran has released more than 400 people detained in anti-government protests that broke out across the country in late December, state-run media reports.

Twenty-one people died and an unknown number were detained during six days of demonstrations, which were the biggest challenge to the Iranian regime since 2009.

Tehran's Public and Revolution courts prosecutor Abbas Jafari Dolatabadi announced Sunday that 440 detainees had been released, Iran's state-run Islamic Republic News Agency (IRNA) reported.

Dolatabadi also urged the judges to pursue the cases of other rioters and prepare the grounds for their immediate release, IRNA said.

Iran's Judiciary Spokesman Gholam Hossein Mohseni Ejei said "only 55 suspects are still under detention in Tehran now" during his weekly press conference on Sunday in Tehran, according to Iran's semi-official Fars News Agency.