Human rights organisations have warned of a growing death toll in Syria as security forces move to quell protests and sit-ins in cities across the country.

As funerals for some of the 27 people shot dead in Deraa on Friday were being held, there were reports of violent clashes between mourners and security services.

An eyewitness in Deraa told the Guardian that security forces had been "shooting randomly" as mass funerals went on. It is not yet clear whether anyone was killed in those clashes. Further funerals are due to take place in Harasta and Douma.

The government issued a warning amid Friday's protests, the largest and most deadly yet.

A statement from the interior ministry, posted by the official Sana media agency, said: "There is no more room for leniency or tolerance in preserving security of the country and protecting the general order."

It added that this would be done "according to the law which specified the conditions for using weapons".

Sit-ins are taking place in Deraa, with the coastal city of Banias cordoned off. The National Organisation for Human Rights said 37 people had died across the country on Friday, including 27 in Deraa, three in Homs and Harasta and one in Douma, bringing the total death toll since 18 March to more than 200.

"We are very worried, especially for Banias where mobile coverage is currently down," Ammar Qurabi, the head of the organisation, speaking from Cairo, said.

Other activists said they were concerned about Latakia, where several districts have been cut off and the army deployed, although the town centre remains quiet.

As in Deraa, there are reports of access to medical care being blocked. "Pharmacists have reported being told by security to shut early and to report those who purchase goods to treat the injured," Wissam Tarif, a local human rights monitor, said.

Both security forces and the shabiha, a group of thugs in Latakia with links to the ruling Assad family, reportedly carried out the violence in Banias and Latakia.

The government has denied this, blaming "spiteful individuals, outsiders and those who were compelled by known foreign parties".

The US president, Barack Obama, has condemned the "abhorrent" crackdown. Activists and protesters have become increasingly frustrated by a muted international response, which has prompted countries in the region to rally to Syrian President Bashar al-Assad's side.

Syrian activists said they were concerned that a cycle of violence had taken hold which would only escalate as the government refused to meet protesters' demands. They said a critical mass of protesters had not yet been reached.

"It is a fight between the people and the regime," Ayman Abdel Nour, a Syrian dissident living in Dubai, said. "Friday was an escalation, but we are still not seeing enough people from all governorates and segments of society coming out to allow them to tip the balance."

However, there are some signs of growing unhappiness among formerly loyal parties. An editor of the state-run newspaper Tishreen, Samira Masalma, was sacked after she criticised shootings by security forces.

There have also been reports of some army and security forces refusing to shoot at protesters. The Guardian could not independently verify these reports.