Iran braced for a fourth day of massive protests Thursday by opponents of President Mahmoud Ahmadinejad in open defiance of the country''s supreme leader, who has urged the nation to unite behind the Islamic state.

Opposition leader Mir Hossein Mousavi urged supporters to wear black Thursday to the planned rally in mourning for the alleged election fraud during Friday''s vote and lives lost during this week''s protests.

Seven demonstrators were shot Monday by pro-regime militia in the first confirmed deaths since the unrest erupted after the election, which the government said Ahmadinejad won in a landslide victory.

Thursday''s protest would be the fourth straight day of major marches in Tehran, including hundreds of thousands of people Monday in a huge procession that recalled the protests of the Islamic Revolution. Mousavi''s Web site said he may join the rally on Thursday to be held at a downtown Tehran square.

Authorities have rounded up perceived dissidents and tried to further muzzle Web sites and other networks used by Mousavi''s backers to share information and send out details of Iran''s crisis after foreign journalists were banned from reporting in the streets.