Iran's supreme leader Ayatollah Ali Khamenei warned Western governments on Monday of a "negative impact" on relations over what he called their meddling in Iran's post-election riots.

Meanwhile, opposition leader Mir Hossein Mousavi on Monday made his first public appearance in a week, saying he was determined to persist in his challenge to the regime, but he stopped short of calling for more street demonstrations after a fierce crackdown on demonstrators.

"We need to make efforts to show our protest ... within the framework of the law," Mousavi said as he welcomed well-wishers at his home for a holiday commemorating Shiism's most important saint, Imam Ali.

"The legitimacy of this government is in question because of the people's lack of trust. This weakens the government from within even if it preserves it in appearance," he said, according to the Iranian news Web site Parsine.

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Mousavi and his supporters claim he was defrauded of victory in the June 12 presidential election, in which Mahmoud Ahmadinejad was re-elected. Massive street protests erupted calling for a new election, but were quashed in a crackdown in which Iranian authorities say 20 people were killed.

Khamenei upheld the results, and the cleric-led leadership has depicted the protests as instigated by foreign government, seeking to erase any lingering doubts about Ahmadinejad's legitimacy.

The supreme leader, who holds final say in all state matters, lashed out on Monday against criticism from abroad in a speech for the holiday.

"Some leaders of Western countries at the level of president, prime minister and foreign minister openly intervened in Iran's internal affairs that had nothing to do with them. Then, they said they don't intervene in Iran's internal affairs," the television quoted Khamenei as telling thousands of Iranians during a ceremony to commemorate a revered Shiite saint.

Khamenei said Iran will pay attention to the remarks and behaviors of Western governments and said that it will definitely have a negative impact on future relations with Iran.

"These governments must be careful of their hostile remarks and behaviors because the Iranian nation will" react, the television quoted Khamenei as saying. "We will calculate the interventionist remarks and behaviors of these governments. Definitely, it will have a negative impact on future relations with the Islamic Republic of Iran."

"As arrogant heads of state know, when the enemy comes near, the people of Iran, despite differences in taste, unite against their enemies into one fist," he said.

Khamenei once again emphasized his support for Ahmadinejad, calling his re-election a "lasting and pure truth."

In France, President Nicolas Sarkozy said his country stands by Britain amid tensions with Iran over the post-election unrest. In a joint news conference with the French leader, British Prime Minister Gordon Brown said Monday that "the Iranian regime must be clear that we will act together with our European partners."

But Brown insisted that the outcome of the Iranian election is a matter for the Iranian people.

In the post-election fallout, Iran detained more than a thousand protesters, along with hundreds of activists, journalists and bloggers. While police say most of the protesters have been released, dozens of prominent political activists remain in jail on vague charges of "acting against national security."

Reformist Web site Norooznews said late Sunday that Hamid Maddah Shourcheh, a member of Mousavi campaign office in the northeastern Iranian city of Mashhad died of torture during detention.

Shourcheh was "martyred as a result of wound inflicted during detention," Norooznews said, adding that he was arrested after holding a sit-in in a Mashhad mosque to protest the election results.

Along with the other arrests, Iranian authorities also detained nine local employees of the British Embassy, leading to vocal protests from Britain and the European Union.

Britain's Foreign Office said Monday that Iran has released another British Embassy staff member, leaving one employee still in custody.

But one staffer, Hossein Rassam, a political analyst at the embassy, still remains in detention, according to his lawyer Abdolsamad Khorramshi. Khorramshi said Saturday that his client has been charged with "acting against national security." Britain has dismissed claims of intervention as baseless.

President Barack Obama's administration, which has made an effort to reach out to Iran since taking office, has left the door open to talks.

In an interview on ABC's "This Week," Vice President Joe Biden Sunday said the U.S. offer to negotiate with Tehran on its nuclear program still stands. Some thought the administration's approach might change in light of the Iranian government's harsh post-election crackdown.

"If the Iranians respond to the offer of engagement, we will engage," Biden said.

Biden also signaled that the U.S. would not stand in the way should Israel opt for military action to eliminate Iran's nuclear facilities, saying Washington "cannot dictate to another sovereign nation what they can and cannot do."

Israel considers Iran its most dangerous adversary and is wary of Ahmadinejad. Israel and the U.S. accuse Iran of seeking to develop weapons under the cover of a nuclear power program, a claim Iran denies.

Iranian police have said that 20 "rioters" were killed during the violence, as well as eight members of the Basij militia tasked with putting down the protests.

The chief of the elite Revolutionary Guards, Gen. Mohammad Ali Jaafari, said in remarks late Sunday that the Guards played the key role in putting an end to street protests, the first admission that the Guards were directly involved in quashing the demonstrators.

The Guards' intervention "gave the revolution new life and strengthened the pillars of the Islamic Republic of Iran," he said, referring to the 1979 Islamic Revolution that overthrew the Shah of Iran, according to the official news agency IRNA.