BEIRUT (AP) -- Syrian security forces fired on anti-government demonstrations across the country on Friday, killing at least 24 people - including several children - as the regime tries to choke off a 9-month-old uprising, activists said.

Some of the worst violence was reported in Homs, a city in central Syria that has emerged as the epicenter of the revolt against President Bashar Assad.

"The earth was shaking," a Homs resident told The Associated Press by telephone, saying explosions and cracks of gunfire erupted in the early morning. "Armored personnel carriers drove through the streets and opened fire randomly with heavy machine guns."

He spoke on condition of anonymity for fear of retribution.

Despite the relentless bloodshed, Assad has refused to buckle to the pressure to step down and has shown no signs of easing his crackdown. The United Nations estimates more than 4,000 people have been killed in the military assault on dissent since March.

Two boys, ages 10 and 12, were hit by stray bullets Friday near government checkpoints in Homs, according to activists. At least two other young teenagers were killed elsewhere, the activists said.

Rami Abdul-Rahman, the head of the British-based Syrian Observatory for Human Rights, said the 10-year-old was shot as he crossed the street in the Bab Sbaaa neighborhood. The 12-year-old was struck as he walked in a crowd exiting a mosque, Abdul-Rahman said.

Anti-government demonstrations traditionally peak after Friday's midday prayers, although witnesses say there appeared to be a concerted effort to prevent any gatherings this week. Troops were deployed heavily and, in many cases, locked down areas before prayers even began.

Security forces also reportedly fired on protests in the Damascus suburbs, the eastern city of Deir el-Zour, Idlbi province near Turkey and elsewhere.

In the southern town of Daraa, activists said telephone and Internet lines were cut.

An activist coalition called the Local Coordinating Committees said up to 35 were killed Friday, most of them in Homs. The Britain-based Observatory had a death count of 24. Casualty tolls are difficult to compile in Syria, where the government has prevented independent reporting and where violence often prevents activists from counting the dead.

Syria has banned most foreign journalists and prevented local reporters from moving freely. Accounts from activists and witnesses, along with amateur videos posted online, provide key channels of information.

Assad is under growing international pressure to curb the bloodshed.

On Wednesday, Assad claimed in a rare interview he never ordered the brutal suppression of the uprising and insisted only a "crazy person" would kill his own people.

Turkey, the Arab League and the European Union have imposed sanctions aimed at squeezing the ailing economy.

On Friday, Turkey urged Assad to punish his security forces and accept an Arab League observer mission if he is "sincere" in his repudiation of violence against civilians.

"If he is sincere he will punish the security forces, he will accept the Arab League observers and help change the atmosphere," Turkey's Foreign Minister Ahmet Davutoglu said Friday. "He still has the opportunity to do this."

Turkey, meanwhile, moved to suspend a 2008 free trade agreement with Syria, which will lead to the imposition of taxes of up to 30 percent on some Syrian goods, authorities said.

The move - like most of the economic measures taken against Syria - is likely to hit the Syrian business class, which until now has been one of the main props of the regime.

Syria already unilaterally suspended the free trade agreement, but Turkey's Cabinet needed to approve the suspension so it can collect the taxes.

Customs and Trade Minister Hayati Yazici said Turkey is planning to encourage Turkish trucks to favor Iraqi and Jordanian routes to Middle East markets, bypassing Syria.

"We are having tensions with Syria," Yazici said. "Of course, our trade is important but our stance based on humanitarian values is above everything."

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Associated Press writers Bassem Mroue in Beirut and Suzan Fraser in Ankara, Turkey, contributed to this report.

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