Iran rally to go ahead - Karoubi aide

"The demonstration plan has not been cancelled and accordingly it must be held this afternoon," said the Karoubi aide, who declined to be named.

Supreme Leader Ayatollah Ali Khamenei issued a strong warning on Friday to leaders of street protests since the June 12 vote that they would be responsible for any bloodshed.

His words appeared to hint at a future crackdown by authorities on rallies after the election, which Khamenei said was fairly won by Mahmoud Ahmadinejad and not rigged, as defeated candidate Mirhossein Mousavi alleges.

At a mass rally in Tehran on Thursday, demonstrators held banners saying they would gather again two days later at around 4 p.m. (1130 GMT).

An ally of Mousavi, Ahmadinejad's main challenger in the election, told Reuters on Friday the moderate politician had not called for people to take to the streets on Saturday or Sunday.

But his supporters may decide to show up anyway, as they did in their tens of thousands on Tuesday despite a call by Mousavi for them to stay home.

If they proceed in defiance of Khamenei's explicit warning, they risk a severe response from security forces, which have so far not tried to prevent Iran's most widespread street rallies since the 1979 Islamic revolution.