Iran has called on the British ambassador to respond to accusations of his government's "interference" in the Islamic Republic, as pro-government rallies continue.
Manouchehr Mottaki, the Iranian foreign minister, told a news conference on Tuesday that the ambassador had been summoned over Britain's interference in Iran's domestic affairs.
"If Britain does not stop talking nonsense it will get a slap in the mouth," he said.
Mahmoud Ahmadinejad, the president, also said that recent opposition rallies in the country are "masquerade" backed by the US and Israel.
The UK said that the envoy would be robust in the face of any Iranian criticism and reiterate that Tehran must respect human rights, Aljazeera.net reports.
Foreign ministry spokesman Ramin Mehmanparast was earlier quoted as saying by the official IRNA news agency that Tehran plans to summon Gass "to elaborate on the Ashura day events."
Deadly clashes erupted on Sunday, when thousands of opposition supporters used the climax of Ashura, one of Shiite Islam's holiest days, to take to the streets for anti-government protests.
"The events which took place on the holy day were an incorrect move and, if any nation encourages the rioters, it constitutes interference in another nation's internal affairs," Mehmanparast added.
"Some Western nations are wrong in their calculations when they compare the movements by a few thousand with Iran's population of tens of millions."
On Monday, Foreign Secretary David Miliband hailed the "great courage" of Iranian opposition supporters who demonstrated during the Ashura rituals, AFP reports.
Meanwhile, Iranian authorities arrested at least a dozen opposition figures on Monday, including former Foreign Minister Ibrahim Yazdi, the human rights activist Emad Baghi and three top aides to the former presidential candidate Mir Hussein Moussavi, Iranian news sites reported.
All told, more than 1,500 people have been arrested nationwide since Sunday, including 1,110 in Tehran and 400 in the central Iranian city of Isfahan, the pro-opposition Jaras Web site reported.
In Hawaii, where he is on vacation, Mr. Obama condemned the violence against protesters and called for the release of those “unjustly detained.”
“For months, the Iranian people have sought nothing more than to exercise their universal rights,” Mr. Obama told reporters. “Each time they have done so, they have been met with the iron fist of brutality, even on solemn occasions and holy days,” The New York Times reports.