Germany and France summoned Iran's ambassadors to express concern over the use of baton-wielding police against demonstrators who say the election was rigged while the United States said Iran must address opposition claims.
And as hundreds of thousands of opposition supporters defied a ban and took to the streets in Tehran, UN Secretary-General Ban Ki-moon said the "genuine" will of the Iranian people should be respected.
"The way the regime responds to legitimate protests will have implications for Iran's relationships with the rest of the world in future," said Britain's Prime Minister Gordon Brown.
"The (Iranian) regime must address the serious questions which have been asked about the conduct of the Iranian elections," he told parliament.
After a meeting in Luxembourg, EU foreign ministers expressed "serious concern" at the weekend crackdown and called for an inquiry into the conduct of the election.
"The actions of the Iranian security forces are completely unacceptable," German Foreign Minister Frank-Walter Steinmeier said on public television.
"France, like its European partners, is awaiting clear answers to the doubts raised over irregularities in the vote," said French foreign ministry spokesman Eric Chevallier in Paris as he explained the decision to summon the ambassador.
In Washington, US Vice President Joe Biden said "there is an awful lot of questions about how this election was run."
"It sure looks like the way they are suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there are some real doubts about that," Biden said.
Iran's official results showed the election was resoundingly won by the hardline Ahmadinejad.
Its supreme leader Ayatollah Ali Khamenei has, however, told the top election supervisory body to probe vote-rigging complaints raised by moderate former premier Mir Hossein Mousavi, Ahmadinejad's defeated challenger.
UN chief Ban said he had taken note of the fact that Khamenei had ordered an investigation into the fraud allegations.
"I am closely following how this investigation will come out," he said, reiterating that "the genuine will of the Iranian people should be reflected and respected in the most transparent, fair and objective manner."
Doubts over the validity of the outcome were also raised by Israel's Defence Minister Ehud Barak. "I'm not sure if the results reflect the real will of the Iranian people," he said.
Barak, whose government believes Iran is trying to acquire a nuclear bomb, said Ahmadinejad's re-election was "bad news".
The Amnesty International rights group called for an investigation into "the shocking scenes of violence meted out by the security forces."
North Korea congratulated Ahmadinejad on his re-election however and wished him success in frustrating foreign interference in his country.
The communist nation's de facto head of state Kim Yong-Nam told Ahmadinejad in a message that his poll victory reflects the Iranian people's support and trust, state media reported.
Kim "sincerely wished him success in his responsible work to frustrate pressure and interference of outsiders and build independent and prosperous Iran", the Korean Central News Agency said.
North Korea and Iran are both engaged in confrontations with the West over their nuclear programmes and are believed to have cooperated in developing long-range ballistic missiles.
Ahmadinejad had been due to attend a summit on Monday in Russia, Tehran's key ally, but sources in the Iranian delegation said he had delayed his trip until Tuesday.
International television networks, meanwhile, reported interference with their reports from Iran on the election violence with an RTVE Spanish television journalist claiming her team had been ordered expelled.
The British Broadcasting Corporation said the satellites it uses to broadcast its Persian-language television service to Iran were being jammed.
