Protests have rocked the Iranian capital since the announcement on Saturday of Mahmoud Ahmadinejad's re-election in the presidential vote. Supporters of Mir Hossein Mousavi, his main challenger, have been rioting on Tehran's streets, prompting a police crackdown. Ahmadinejad held a "victory rally" on Sunday, defending the election, while Mousavi appealed for both calm and cancellation of the vote. Al Jazeera's Teymour Nabili reports from Tehran against a backdrop of rising political tension.