Iranian security forces in Tehran killed at least eight people, including a nephew of opposition leader Mir Hossein Mousavi, during the fiercest clashes with anti-government protesters in months.
Amateur video footage showed an enraged crowd carrying away one of the casualties, chanting: 'I'll kill, I'll kill the one who killed my brother.'
In several locations in the Iranian capital demonstrators fought back against security forces, hurling stones and setting motorcycles, cars and vans on fire, according to video footage and pro-reform websites.
Demonstrations also took place in at least three other cities.
A close aide to Mousavi, a presidential contender in the disputed June election, said his 35-year-old nephew Ali Habibi Mousavi Khamene died of his injuries in a Tehran hospital.
Mousavi's website and another reformist website also said Ali Mousavi died during clashes in which security forces reportedly fired on demonstrators.
The protesters in Tehran tried to cut off roads by burning barricades that filled the sky with billowing black smoke.
Opposition activists have held a series of anti-government protests since the death of a dissident cleric, Grand Ayatollah Hossein Ali Montazeri, last week.
They then opened fire directly at protesters, killing at least three people, according to witnesses. A fourth protester was shot dead on a nearby street, they said.
Protesters say president Ahmadinejad won the election through massive vote fraud and that Mousavi was the winner.
Tehran's police chief denied that his officers had fired on the crowd - or that they were even armed.
Foreign media organisations were barred from covering the demonstrations.
Mobile phone services were down and Internet connections were slowed to a crawl, as has happened during most other days of opposition protest in an apparent government attempt to limit attention on the events.
Opposition activists have held a series of anti-government protests since the death of a dissident cleric last week.
His memorials have brought out not only the young, urban activists who filled the ranks of earlier protests, but also older, more religious Iranians who revered Montazeri on grounds of faith as much as politics.
Tens of thousands marched in his funeral procession in the holy city of Qom on Monday, many chanting slogans against the government.
