Renewed violence across Syria has left dozens dead, as Arab League observers continue their mission to oversee a peace plan for the country.
In the capital Damascus, state TV said up to 26 people died when a suicide bomber detonated explosives near a bus.
Opposition groups say the government staged the blast to influence the Arab League mission.
Elsewhere in Syria, activists say 35 people have been killed by security forces during anti-government protests.
The Damascus blast happened at a busy junction in the Midan district and state TV showed the shattered blood-stained windows of what appeared to be a bus carrying policemen.
Authorities say most of those killed were civilians, but some security personnel were among the casualties.
Syria's interior ministry said it would "strike back with an iron fist" to what it called "terrorist escalation".
First reports said 10 had been confirmed dead, but Interior Minister Ibrahim al-Shaar later put the toll at 26, saying 15 bodies had been too badly mutilated to be identified, state TV reported.
State news agency Sana released images of an Arab League observer being shown dead bodies lying on the floor of a hospital.
Most foreign correspondents have been barred from reporting within Syria.
However, one foreign journalist who visited the scene of the blast, Ian Black of the UK's Guardian, told the BBC's Newshour programme that body parts were strewn around.
"They were being shown very energetically. Somebody held up a glove filled with blood and said 'this is Syrian blood'. There was a sense that the Syrian authorities wanted to show what had happened."
He added: "What we didn't see were any other bodies of the people who are said to have been killed. We left with the sense of this is a horrific spectacle but that maybe some of the details of it weren't quite as one might have expected."
The explosion also damaged a nearby police station, witnesses said.
Mr Shaar said a suicide bomber "detonated himself with the aim of killing the largest number of people".
Two weeks ago 44 people died in similar blasts also blamed on terrorists. Opposition groups accused the government of staging them.
A spokesman from the opposition Syrian Revolution General Commission (SRGC), inside Syria, told the BBC they had "nothing to do with the explosion".
He blamed Syrian security forces, adding: "The regime is sacrificing innocent people to misguide the world and create this picture that they are fighting terrorists."
Col Riad al-Asaad, leader of the Free Syrian Army - the main armed group fighting the regime - reportedly denied involvement in Friday's attack in a TV interview from his base in Turkey.
Arab League monitors are in Syria on a month-long observer mission trying to ensure compliance with a peace plan. However, activists say a Syrian government crackdown has continued, with scores of people killed.
French Foreign Minister Alain Juppe said on Friday that the mission was "not at present able to do its job properly", AFP news agency reported.
Burhan Ghalioun - the head of the main opposition group, the Syrian National Council - told the BBC he feared the observers could be providing political cover for the regime to suppress street protests.
Opposition activists have urged Syrians to take to the streets in mass protests ahead of an Arab League meeting in Cairo on Sunday, which will debate the initial findings of the observer mission.
Meanwhile, activists reported further violence on Friday. Anti-government protests have regularly followed traditional Friday prayers.
The Local Co-ordination Committees said nine protesters had been killed in Hama, 14 in the suburbs of Damascus, eight in Homs, three in Idlib and one in Deraa.
The numbers cannot be verified.
Sana news agency also reported that an oil pipeline between Hama and Idlib in the north-west had been blown up by a "terrorist group".
The UN says more than 5,000 civilians have been killed since protests against President Bashar al-Assad began 10 months ago.