At least 28 people have been killed and 235 wounded in two bombs targeting security compounds in Syria's second city of Aleppo, state media report.
State television said the death toll included both civilians and members of the security forces and blamed "armed terrorist gangs" for the attacks.
Within minutes, it broadcast footage showing corpses and mangled body parts.
The rebel Free Syrian Army said it was operating in the area at the time, but was not responsible for the blasts.
Col Malik al-Kurdi, the FSA's deputy leader, told BBC Arabic that it had been monitoring the activity of security forces personnel and members of the pro-government Shabiha militia inside a Military Intelligence compound and a riot police base in Aleppo on Friday morning.
"When they were gathering in a square to go to the mosques and repress demonstrations, two groups from the FSA targeted the two buildings with small arms and rocket-propelled grenade fire," he said.
"After violent clashes, there was an explosion inside the Military Intelligence building. At first we didn't know what it was, but we think it was the regime trying to stop the operation of the FSA," he added.
Another FSA spokesman, Col Mahir Nouaimi, told AFP: "This criminal regime is killing our children in Homs and carrying out bomb attacks in Aleppo to steer attention away from what it is doing in Homs."
Opposition members also blamed the government, accusing it of trying to discredit the uprising.
Izzedine al-Halabi, an activist in Aleppo, told the BBC there had been suspicious activity by security personnel who sealed off the area around the main intelligence compound shortly before the blasts.
"We hold the Syrian regime entirely responsible for this explosion," he said.
A weeping Syrian state TV reporter said the bomb targeting the Military Intelligence compound went off near a park, where people had gathered for breakfast and children had been playing.
Some children were killed in the blast, he said, holding up an inline-skate.
Bulldozers could be seen in the TV footage clearing debris that filled the street, and nearby buildings appeared to have had their windows shattered.
"Civilians and members of the military were martyred and wounded in the terrorist explosions,'' the channel reported.
The channel showed similar footage from the site of the second explosion, which the reporter said was the result of a suicide car bombing.
The blast left a crater several metres wide in the road and blew a lorry onto its side.
Emergency workers were shown holding up body parts which they placed in black bin bags.
Aleppo has seen only minor protests and relatively little violence since the uprising against President al-Assad erupted in March, which human rights groups say has left more than 7,000 civilians dead.
On 6 January, 26 people were killed in what officials said was a suicide bombing in Damascus.
Two weeks earlier, 44 reportedly died in twin suicide bomb attacks targeting security compounds in the capital.
Later on Friday, the Local Co-ordination Committees, an activist group that organises and documents protests, said 12 people had been killed by security forces and Shabiha militiamen at a protest in the Marjeh district of Aleppo.
It said another 22 people had been killed nationwide, including 10 in the central city of Homs and eight in the Damascus suburb of Domair.
Meanwhile, residents of the central city of Homs say tanks are massed outside several opposition-held districts.
Overnight, tanks entered the district of Inshaat, next to the protest centre of Baba Amr, the Syrian Observatory for Human Rights said.
There was also sporadic shelling and gunfire throughout the city on Friday. At least four people were reportedly killed in Baba Amr.
Activists say the intense bombardment of many parts of Homs by security forces since Saturday has left more than 400 people dead. US President Barack Obama has condemned the "outrageous bloodshed".
The opposition called for nationwide protests on Friday to denounce Russia's veto of a UN Security Council resolution demanding President Bashar al-Assad's government stop killing its own people.
Russia's Deputy Foreign Minister Sergei Ryabkov said the opposition "bore full responsibility" because it had refused to begin talks with the government and accused Western powers of being "accomplices".
But Saudi Arabia's King Abdullah criticised Russia's support of Mr Assad.
"There is no doubt that the confidence of the world in the United Nations has been shaken," he said on Saudi state TV on Friday. "Unfortunately, what happened in the UN, in my opinion, is an unfavourable initiative."