Two suicide car bombers struck security compounds in Aleppo, killing 28 people, Syrian officials have said the first significant violence in an industrial center that has largely stood by President Bashar Assad during the 11-month uprising against his rule.

Anti-Assad activists denied any involvement and accused the regime of setting off Friday’s blasts to smear the opposition as government forces pummel rebels in one of their main strongholds, Homs. State media touted the bombings as proof the regime faces a campaign by terrorists, not a popular uprising.

The military stepped up its siege of Homs, where hundreds have reportedly been killed over the past week. Soldiers who have been bombarding the central city made their first ground move, storming into one of the most restive neighborhoods.

Satellite image provider DigitalGlobe Inc., based in Colorado, released photos on Friday that it said show Syrian army tanks and other armored vehicles near apartment buildings in the city.

Troops and security forces opened fire on anti-regime protesters who streamed out of mosques after Friday prayers nationwide. At least 27 civilians were killed, according to the London-based Syrian Observatory for Human Rights.

The morning blasts in Aleppo, Syria’s most populous city, ripped apart the facades of the local headquarters of the Military Intelligence Directorate and a barracks of the Security Preservation forces.

At the Directorate, windows were shattered and a large crater was torn into the pavement outside the entrance. A weeping correspondent on state-run TV showed graphic footage of at least five corpses, collected in sacks and under blankets by the side of the road.

Security officials said suicide bombers in explosives-packed vehicles tried to smash through the entrances of both sites. At the barracks, Brig. Firas Abbas told an Associated Press reporter on a government-guided visit to the scene that the vehicle made it through one roadblock before detonating near the gates. A head lay on the ground and security officials said it belonged to a suicide attacker.

State television cited the Health Ministry as saying 28 people were killed in the two blasts and 235 wounded. The dead included 11 security personnel killed at the barracks, 13 military personnel killed at the military intelligence building and four civilians, security officials said.

State TV blamed ”terrorists.” But anti-government activists accused the regime of setting off the blasts to discredit the opposition and avert protests that had been planned in the city on Friday.

Capt. Ammar al-Wawi of the Free Syrian Army, a rebel group that wants to bring down the regime by force, denied involvement. He said fighters from his group had a short gunbattle with troops several hundred yards (meters) from the Directorate about an hour before the explosion but did not carry out the bombings.

”This explosion is the work of the regime to divert world attention from the crimes it is committing against the people of Homs,” he said.

The blasts were the fourth such dramatic suicide attack since late December. All occurred on Friday mornings against various security headquarters and prompted the same exchange of accusations. The earlier attacks, in the capital Damascus, killed dozens of security forces and civilians, according to Syrian officials. Nobody has claimed responsibility for any of the attacks.

Friday’s bombings were the first significant violence in Aleppo, a northern city of some 2 million people that is home to a prosperous business community and merchant class whose continued backing for Assad has been crucial in bolstering his regime.

The city has seen only occasional protests. Assad’s opponents have had little success in galvanizing support there, in part because business leaders have long traded political freedoms for economic privileges. Also, the city has a large population of ‘Kurds’, who have mostly stayed on the sidelines of the uprising since Assad’s regime began giving them long-denied citizenship as a gesture to win support.

Still, hours after the explosions, hundreds of protesters marched in several Aleppo neighborhoods after Friday prayers, part of nationwide demonstrations labeled ”Friday of ‘Russia is killing our children”’ a denunciation of Russia’s veto of a U.N. attempt to condemn Syria’s crackdown.

Regime forces opened fire on the Aleppo protesters, killing at least seven, according to the Observatory. Another activist group, the Local Coordination Committees put the Aleppo toll at 12 and said 22 others had been killed nationwide. The figures could not be independently confirmed due to restrictions the Syrian government has put on journalists.

Assad’s crackdown has killed well over 5,400 people since the uprising began in March, according to U.N. estimates.

The regime’s crackdown on dissent has left it almost completely isolated internationally except for key support from Russia and China.