The stadium, which holds about 25,000 people, reportedly was filled to capacity for today's rally.
Both chambers of Afghanistan's parliament -- dominated by alleged war criminals -- have passed the bill.
A spokesman for Afghan President Hamid Karzai has said he is unlikely to sign the legislation.
But according to Afghanistan's 2004 constitution, the bill would be deemed as law if the president does not take action on it within 15 days. If the Afghan president rejects a bill, the lower chamber of parliament can override his veto with a two-thirds majority vote.
Human rights groups and the UN Assistance Mission in Afghanistan (UNAMA) say such a bill does not protect alleged war criminals from prosecution under international law.
(with material from AP, AFP)