Supporters of Iranian presidential candidate Mir Hossein Mousavi have come out for a second mass demonstration against President Ahmadinejad, according to reports.
Mr Mousavi himself warned his supporters not to attend the rally for fear that it might lead to violent clashes with a rival demonstration being held in support of Mr Ahmadinejad.
Mr Ahmadinejad was declared the winner of last Friday's presidential election by a margin of two to run. However his opponents have accused him of vote rigging.
According to the reports the crowds of opposition supporters are bigger than those seen on Monday, which were themselves the largest since the Iranian Revolution 30 years ago.
The opposition rally is being held in the north of Tehran where the opposition is particularly strong.
The pro-Ahmadinajad rally is taking place in the middle of Tehran and is reported to have transported supporters from outside the city to boost numbers.
Reports are sketchy due to strict new restrictions on the media in the country.