Shoppers walk past a defaced poster of leading challenger and reformist candidate Mir Hossein Mousavi in Tadrish bazaar in northern Tehran. (Ben Curtis, File/Associated Press)

Reformist leader Mir Hossein Mousavi vowed Thursday to continue to press for an annulment of Iran's presidential election results even as authorities detained 70 university professors who had met with him.

Mousavi alleges electoral fraud occurred during the June 12 election that handed President Mahmoud Ahmadinejad victory by a 2-to-1 margin.

Mousavi said he is coming under increasing pressure to withdraw his challenges before Iran's Guardian Council regarding the election results.

"I am not ready to withdraw from demanding the rights of the Iranian people," Mousavi said in a statement posted on his website.

Iranian security forces on motorcycles are seen amid traffic near the parliament building in Tehran on Wednesday. (Demotix Images, Hamed/Associated Press)

Despite acknowledging voting irregularities — more ballots cast than voters registered — had occurred in several Iranian districts, the 12-member cleric council has said there were no major incidents of fraud and there will not be a new vote.

According to government officials, Ahmadinejad won 62.6 per cent of the votes, while reformist candidate Mousavi garnered 33.75 per cent in a contest that appeared to divide urban and rural voters.

Mousavi said Iranians have a right to express "their opposition to what happened in the election and after that."

The election's outcome has spurred allegations of vote-rigging and sent tens of thousands of protesters from both sides into the streets.

Mousavi said authorities are increasingly trying to isolate and vilify him and his supporters.

70 university professors detained

Seventy university professors have reportedly been detained as Iran widens its crackdown on opposition demonstrators.

The academics met with Mousavi on Wednesday and were detained immediately following the meeting, according to a website affiliated with the defeated presidential candidate.

The detainees have been pushing for a more liberal form of government in Iran, according to the report on the website.

The report could not be confirmed. The regime's restrictions on reporters have made it extremely difficult to get independent verification of reports of violence and detentions.

At least 240 protesters and activists are believed to have been taken into custody since the June 12 vote, according to Western-based human rights groups.

Day of mourning cancelled

Iran's Supreme Leader Ayatollah Ali Khamenei and the country's elite military force, the Revolutionary Guard, have warned against continued demonstrations and the government has set up a special court to deal with detainees from the rallies.

On Wednesday, reports from Tehran suggested thousands of security officers clamped down on several hundred protesters who had gathered at the parliament buildings.

At least 17 people have been killed in the protests following the election as authorities intensified their crackdown.

A rally scheduled for Thursday to mourn the dead protesters was cancelled amid the continued crackdown. Organizers said authorities had not granted permission for the public gathering.

The most senior dissident cleric in Iran, Ayatollah Hossein Ali Montazeri, warned the authorities against trying to snuff out opposition voices.

If people are not allowed to voice their demands in peaceful gatherings, it "could destroy the foundation of any government," regardless of its power, Montazeri said, calling for a neutral body to investigate the claims of election fraud.

U.S. invitations revoked

Division about the election results also appeared among legislators, with several Tehran newspapers reporting Thursday that only 105 of 290 members of parliament attended a victory celebration for Ahmadinejad.

Meanwhile Ahmadinejad called on U.S. President Barack Obama on Thursday not to interfere in Iran's internal affairs.

Obama has stated that he is "appalled and outraged" by post-election violence in Iran. For the first time in 30 years, the U.S. has also not issued invitations to Iranian diplomats to attend its annual July 4 celebrations.

"Why has Mr. Obama, who advocates change, been trapped and follows the same path as Bush?" state television quoted Ahmadinejad as saying.

The move to withdraw the invitations was largely symbolic, as no Iranian diplomats had even responded.

With files from The Associated Press