The newspaper said the Iranians were advising the Syrian government and providing hardware, but were not engaged in any physical attacks on protesters.
The Guardian said it got the information from an unnamed Western diplomat based in Damascus.
The diplomat said there was a “significant” increase in the number of Iranian personnel in Syria since the protests began in mid-March.
The newspaper quoted the diplomat as saying mass arrests in door-to-door raids, similar to those that helped to crush Iran’s “green revolution” in 2009, had been stepped up in the past week. The mass arrest tactic in Tehran, however, was not an Iranian invention, so it is not proof of Iranian involvement in Damascus.
Syrian human rights groups say more than 7,000 people have been detained since the uprising began and more than 800 people have died, making it the bloodiest uprising this year in the Arab world after the civil war in Libya.
“Tehran has upped the level of technical support and personnel support from the Iranian Republican Guard [Pasdaran] to strengthen Syria’s ability to deal with protesters,” the diplomat said. “Since the start of the uprising, the Iranian regime has been worried about losing its most important ally in the Arab world and important conduit for weapons to Hezbollah [in Lebanon],” the diplomat said.
Last month, White House officials made allegations about Iranian assistance for the regime. They emphasized that Iranians were helping the Syrian government intercept or block Internet, mobile phone and social media communications between the protesters and the outside world.
Some web reports have asserted that Iranian troops were in Syria knocking the heads of the protesters. But both the White House and The Guardian’s source said the Pasdaran were not on the street confronting protesters.
Activists and diplomats say Iran’s assistance includes help to monitor Internet communications such as Skype, widely used by Syrian activists, advice on methods of crowd control, and providing equipment such as batons and riot police helmets.
In 1978, when the Iranian revolution erupted, the police and military there had only limited training in riot control and bungled efforts to smother the protests then. Since the revolution, the Islamic Republic has put a very heavy emphasis on crowd control and riot suppression techniques.
Syria has denied seeking or receiving assistance from Iran to put down the unrest.
In a statement issued Friday, Iran’s Foreign Ministry stressed Syria’s “prime role” in opposing Israel and the US, and urged opposing forces in the country to compromise on political reform. US policy on Syria was based on “opportunism in support of the Zionist regime’s avarice,” it said.
The ruling Assad family is from the minority Alawite sect that is Shiite. It is likely to be nervous at being seen calling on Shias to help stifle protests. The country is 74 percent Sunni, 13 percent Shia, 10 percent Christian and 3 percent Druze.