Opposition member Mehdi Karoubi said the government faced pressure from members of parliament, the Iranian public and the rest of the world.
Speaking to the BBC in Tehran, he repeated allegations over the abuse of anti-government protesters in prison.
President Mahmoud Ahmadinejad was re-elected in disputed elections in June.
"From the first day the election result came out I was convinced that Mr Ahmadinejad would not survive the full four years of his term," Mr Karoubi said.
"Even within the last six months, the government has only been kept in power by force."
Widespread protests erupted in Iran after the election, with hundreds of anti-government demonstrators arrested.
In a rare interview with a foreign broadcaster, Mr Karoubi told the BBC that heavy-handed treatment by the authorities had been counter-productive.
"At the beginning of the movement, it was just a protest against the election, we didn't recognise it as a serious election.
"The government's response, the crackdown, has not calmed things down at all. In fact, it's just made the chanting louder. It's just increased the people's demands."
He stood by his allegations that opposition activists have been raped in detention - a charge vehemently denied by authorities.
He said he was not afraid of being prosecuted over the claims as threatened by several government officials.
Mr Karoubi was interviewed by the BBC at his home in Tehran as his newspaper has been closed down, and all party meetings banned.
"We can't even get permission for any social gatherings," he said.
Although he said he did not believe he was about to be arrested, the government seems to be edging closer to taking that radical step, as the confrontation with the opposition threatens to erupt once again, says the BBC's Tehran correspondent Jon Leyne.
With the BBC blamed by the Iranian government for stirring up trouble after the election, Mr Karoubi's interview was a deliberate act of defiance, our correspondent adds.
Six months on from the election, there have been more demonstrations in Iran.
At the weekend, Iran's Supreme Leader accused the opposition of breaking the law by insulting the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.
The remarks centre on an alleged incident last Monday during which a poster of Imam Khomeini was torn up.
Opposition leaders say the alleged incident - shown on state television - was doctored.