The funeral of a nuclear scientist killed in a targeted car bomb attack has taken place in Tehran.
Thousands attended Mostafa Ahmadi-Roshan's funeral, state TV reported.
His death has further stoked tensions between Iran and the US and its allies who fear Iran is trying to develop nuclear weapons.
Russia has warned that new sanctions and any military action against Iran over its nuclear programme would be seen as attempted regime change.
Iran has blamed the US and Israel for the killing of the 32-year-old scientist, allegations the US has denied. Israel has not commented on the incident.
Iran's supreme leader Ayatollah Ali Khamenei has vowed to punish those behind the death of Mr Ahmadi-Roshan, who worked at the Natanz uranium enrichment facility.
"We will continue our path with strong will... and certainly we will not neglect punishing those responsible for this act," Mr Khamenei was quoted as saying by Iran's official Irna news agency.
Iran has called on the UN to investigate Mr Ahmadi-Roshan's death, according to Iran's Press TV.
US Secretary of State Hillary Clinton has "categorically" denied US involvement in the bombing though Defence Secretary Leon Panetta has admitted: "We have some ideas as to who might be involved."
Tehran said the death would not impede "progress" in its nuclear programme.
Washington and its allies suspect Iran of secretly trying to develop a nuclear weapons capacity but Iran insists the programme is peaceful.
Russia has warned that Israel is pushing for war with Iran.
"There is a likelihood of military escalation of the conflict, towards which Israel is pushing the Americans," Kremlin Security Council head Nikolai Patrushev was quoted as telling Russia's Interfax news agency, AFP reports.
Russia's Deputy Foreign Minister Gennady Gatilov said this would be seen as direct intervention.
"Additional sanctions against Iran, as well as potentially any military strikes against it, will unquestionably be perceived by the international community as an attempt at changing the regime in Iran," Mr Gatilov said.
Western nations are imposing sanctions on Iran over its nuclear programme.
Japan's foreign minister has said that the imposition of any additional sanctions against Iran would have to be weighed carefully to ensure they prove effective.
Koichiro Gemba said that measures which caused an oil price rise would be counterproductive.
Correspondents say his remarks appear to backtrack on a pledge made on Thursday by Japan's finance minister to begin reducing Iranian oil imports.
In response to the sanctions, Tehran has threatened to block the transport of oil through the Strait of Hormuz, a threat which Ms Clinton has described as "provocative and dangerous".
On Friday, diplomats said a high-level International Atomic Energy Agency (IAEA) delegation would visit the country in late January, to try and soothe relations between Iran and the US, AFP reports.
The UN's nuclear watchdog recently confirmed that Tehran had begun enriching uranium up to 20% at its underground northern Fordo plant.
Mr Ahmadi-Roshan died immediately when, according to Iranian news sources, two men on a motorcycle attached a magnetic bomb to his car and detonated it.
Protests planned for Thursday outside the French, German and British embassies would be held after Friday's funeral, according to Fars.