An Iranian university professor killed last week by a blast from a remote-controlled bomb strapped to a parked motorcycle may have been the victim of an Arab hitman, according to opposition groups.

The murder of Masoud Ali Mohammadi, 50, a supporter of Mir Hossein Mousavi, the opposition leader, has been blamed by the Tehran regime on "mercenaries" financed by Israel and Washington because of his role as a nuclear physicist.

However, opposition groups who monitor Hezbollah, the militant Lebanese movement, in Tehran, claim that a member of the group, known by his pseudonym "Abu Nasser", was photographed at the scene of the explosion in Tehran's affluent Gheytarih suburb.

A German-based opposition group released a photograph of a man of similar appearance who, it alleges, was one of the pro-regime demonstrators who stormed Mousavi's office in Tehran after disputed presidential elections last June.

The opposition claims the Revolutionary Guard uses Hezbollah operatives for some bloodthirsty tasks because they have a reputation for ruthlessness, and are outsiders and can always be blamed as opposition sympathisers.

Tehran has gone to great lengths to suggest that Mohammadi was killed because he was a nuclear scientist, implying that he was part of Iran's programme to develop nuclear weapons.

However, Majid Mohammadi, a visiting scholar at the Institute for Global Studies at Stony Brook University, New York, and also a friend, said: "He was not a nuclear physicist. He was just a physicist. I believe the Iranian [official] media highlight this word ‘nuclear' to imply he was killed by the Israelis or Americans."

Iran maintains close links with Hezbollah in southern Lebanon, which it supplies with weapons and funds. Scores of Hezbollah officials are based in Tehran and, according to the opposition, are frequently used by the regime to crack down on its opponents.

It alleges that Abu Nasser had been spotted alongside revolutionary guards in recent anti-government demonstrations. Sources in Tehran suggest the murdered scientist was a strong supporter of the opposition and his assassination was a warning by the regime to its opponents.

Last week President Mahmoud Ahmadinejad said that the "method of bombing" indicated "Zionists did it and that they hate us and don't want to progress".

In Israel there was no official comment. In recent years Mossad, the Israeli overseas intelligence service, has launched extensive undercover operations inside Iran to attempt to derail its nuclear ambitions.

Mysterious fates have befallen a number of Iranians involved in the nuclear programme. Two years ago a nuclear scientist was found dead at his home, apparently from carbon monoxide poisoning.

Mishka Ben-David, a former Mossad agent, doubts that Israel was involved in last week's assassination: "Could you imagine how much risk it takes to carry out an attack of that kind in Tehran?" He does not believe the Iranian professor was worth the effort.