Austrian prosecutors are investigating allegations that Iran's president-elect was involved in the 1989 assassination of a Kurdish leader in Vienna.
Mr Ahmadinejad denies the allegations
Austrian politician Peter Pilz said there was "credible evidence" to link Mahmoud Ahmadinejad to the murder of Iranian exile Abdul Rahman Ghassemlou.
The president-elect has dismissed the allegations as "baseless".
The Iranian foreign ministry has summoned the Austrian ambassador to Tehran to demand an explanation.
Mr Ahmadinejad, a hardline conservative, was the surprise victor in last month's Iranian presidential election. Previously, he was the mayor of Tehran.
Mr Ghassemlou, a Kurdish opposition leader, and two of his colleagues were shot dead on 13 July 1989, during a visit to Vienna.
Austrian prosecutors said they were acting on new information on the killings provided by Mr Pilz, of Austria's opposition Green Party, who has been investigating the case for some years.
They said they wanted to question several witnesses now believed to be in France.
"I cannot personally say whether the allegations... are true, but I can say that they are credible," Mr Pilz said.
An Iranian spokesman said: "These accusations are ridiculous and without foundation and, for this reason, we have summoned the Austrian ambassador to the ministry of foreign affairs to demand an explanation."
Meanwhile the US is examining separate allegations that Mr Ahmadinejad took part in the hostage-taking at the US embassy in Tehran in 1979.
Mr Ahmadinejad has strongly denied his involvement, and several hostage-takers have also denied that he was among them.
The allegations surfaced after some former hostages said they recognised Mr Ahmadinejad following his election victory.
Photographs have appeared on the internet showing a young bearded man leading a blindfolded American hostage - alleging that this was Mr Ahmadinejad a quarter of a century ago.
But the man in the photograph appears much taller than Mr Ahmadinejad, and looks nothing like other pictures of him as a student which can be found on his website, says the BBC's Frances Harrison in Tehran.