STOCKHOLM (Reuters) -Sweden's government is considering changing the Public Order Act to make it possible for police to deny permission for acts such as burning the Koran but only if they threaten national security, it said on Friday.

Sweden raised its terrorist alert to the second highest level on Thursday, saying it had thwarted attacks after Koran burnings and other acts against Islam's holiest text outraged Muslims and triggered threats from jihadists.

Insults towards public figures or against religions are protected by Sweden's far-reaching freedom of speech laws and the government rules out changing them.

However, Minister of Justice Gunnar Strommer said on Friday he would appoint a commission to look into giving police wider powers to deny acts such as Koran burnings.

"Of course, general international dissatisfaction or vague threat should not be enough – it must be about serious and qualified threats," Strommer told a news conference.

He added it could give police the power to select a different location for a protest or to dissolve it.

An Iraqi living in Sweden has damaged several copies of the Koran in recent months. Many Muslims view desecrating the Koran, which they see as the literal word of God, as a grave offence. A media outlet linked to militant group al Qaeda has urged violent retribution against Sweden.

The decision to appoint a commission met with immediate scepticism from several political parties, including the government's support party, the anti-immigration Sweden Democrats.

"Even if different values always need to be weighed against each other, the Sweden Democrats will never accept that we adapt to threats and pressure from Islamists and dictatorships," Sweden Democrats' party leader Jimmie Akesson said in a statement.

Earlier on Friday, the government said it had tightened security at embassies and other missions due to an increase in threats against Swedish interests abroad.

Sweden's Foreign Minister Tobias Billstrom told TT that Sweden has increased security at embassies and other missions, without giving detail for security reasons.

"But the safety of the Ministry of Foreign Affairs' staff is the highest priority," he said, adding that the safety of families of diplomats and local staff was also taken into account.

"So there are different categories that are affected by this security work that is now being intensified," he said.

Billstrom did not reply to a request for comment.

(Reporting by Johan Ahlander; Editing by Angus MacSwan and Barbara Lewis)