Punitive international sanctions imposed on Iran have strengthened Mahmoud Ahmadinejad's government and assisted its post-election crackdown on the opposition Green movement, the leading reformist politician and former presidential candidate Mehdi Karroubi has told the Guardian.

In his first interview with a British newspaper since widespread unrest erupted after Ahmadinejad's disputed re-election as president last June, Karroubi blamed the US and Britain for adopting counterproductive policies to combat Iran's suspect nuclear programme, describing sanctions as a gift to the Iranian regime.

"These sanctions have given an excuse to the Iranian government to suppress the opposition by blaming them for the unstable situation of the country," Karroubi said in email responses to the Guardian.

Karroubi, 73, a former speaker of the Majlis, the Iranian parliament, under the reformist president Mohammad Khatami, and a candidate in last year's election, said that isolating Iran would not bring democracy. "Look at Cuba and North Korea," he said. "Have sanctions brought democracy to their people? They have just made them more isolated and given them the opportunity to crack down on their opposition without bothering themselves about the international attention."

The UN security council agreed a new round of sanctions on Iran in June after the US and Britain, which believe Iran is secretly developing nuclear weapons, obtained backing from Russia and China. The EU, and individual countries such as the US and Britain, subsequently imposed additional punitive measures.

The move followed Washington's rejection of a proposed deal, brokered by Brazil and Turkey, under which Iran would have handed over nearly half of its stock of low-enriched uranium in return for "safe" nuclear fuel supplies that could not be used in bomb-making. Turkey and Brazil voted against the new UN sanctions, but today Brazil announced that it was reluctantly prepared to enforce them.

Mir Hossein Mousavi, who helped lead the protests following last summer's election, co-authored a public letter with Karroubi last week in which they condemned the sanctions while blaming Ahmadinejad's government for mishandling negotiations over Iran's nuclear programme.

"Sanctions have targeted the most vulnerable social classes of Iran including workers and farmers," the letter said.

Karroubi told the Guardian: "On the one hand, the government's mishandling of the economy has resulted in deep recession and rising inflation inside the country, which has crippled the people of Iran and resulted in the closure of numerous factories. On the other hand, we have sanctions which are strengthening the illegitimate government."

Karroubi, who was imprisoned before the Islamic revolution in 1979,said that despite widespread corruption, the shah's regime treated its opponents less harshly than the current government, partly because the shah was sensitive to international criticism.

"But because Iran is getting more isolated, more and more they [Ahmadinejad's government] are becoming indifferent to what the world is thinking about them," he said.

Last summer's unrest resulted in the killing, beating or arrest of hundreds of protesters who took to the streets convinced that Ahmadinejad, who is backed by Iran's supreme leader, Ayatollah Ali Khamenei, had stolen the election.

Hopes the regime would call a fresh vote or collapse under public anger dissipated in clouds of teargas, counter-demonstrations organised by the government, and often brutal repression. Since then, Iranians have suffered a crackdown on dissent and an increase in human rights abuses.

Yesterday the US secretary of state, Hillary Clinton, issued an appeal to Iran to honour its international treaty obligations to respect the rights of its citizens.

She also expressed concern about the case of Sakineh Mohammadi Ashtiani, who was sentenced to death by stoning after being found guilty of adultery. In his interview with the Guardian, Karroubi condemned Ashtiani's sentence and said he was opposed to stoning in principle.

Karroubi said that since the election his office has been sealed off and his newspaper, Etemad-e-Melli (the National Trust), closed down while he was under informal house arrest. Wherever he went, he said, groups of government supporters, sometimes accompanied by plainclothes basiji militiamen, followed him.

"In the last year, they [officials] have tried to suppress me in many ways," he said. "Once I was physically attacked, on the anniversary of the Islamic revolution, and my son Ali was arrested and severely tortured. During a recent visit I had to Qazvin province, they went further in attacking me and opened fire on my car and later raided my house."

Ayatollah Khamenei has never attacked him or Mousavi by name, but always referred to them as "leaders of sedition", a term now now routinely used to describe opposition leaders. Last month, during a visit to Qom, Karroubi was met by government supporters shouting "western stooge".

Despite having to call off protests in the face of the government crackdown, Karroubi said he believed the Green movement had not been defeated. "It's no longer possible for the opposition movement to pour out en masse into the streets … But we also do not think it's necessary any more to do this," he said.

The movement's message had already reached the world, he said. "People were out in the streets to inform the world of what is really happening inside Iran, and they succeeded in doing so. Now the world knows what is the problem in Iran."

Karroubi said he still believed in the Islamic republic, but not the current ruling system. "I should make it clear that we are a reformist movement, not a revolutionary one … We are seeking nothing more than a free election."

Asked about criticism that the opposition has no clear leader and sometimes appeared divided, Karroubi said: "In my opinion, it's an advantage that no specific person is the leader. I think that the only reason the Green movement has not been stopped yet is because it doesn't have one leader or unified leadership. If it had, then by arresting that leader they could have controlled the whole movement."

Reaching out to Iran's ordinary people remained the opposition's biggest problem: any newspaper that mentioned the Green movement would be immediately closed down, he said. "We are not even allowed to publish a funeral announcement at the moment."