State-backed Syrian militias and security forces are roaming hospitals looking for demonstrators and commandeering ambulances to abduct them, medical workers inside the country say.

The workers – including doctors, ambulance drivers, medical students and Red Crescent volunteers – claim they face intimidation from regime officials who are trying to prevent them from treating wounded dissidents.

The allegations centre on the battle-torn city of Homs, where security forces and defectors were involved in heavy clashes . At least 19 people are believed to have been killed in the fighting, which was among the most intensive of the past two months.

"They are chasing defectors," a Homs resident who called himself Fadi told the Guardian by Skype. "And the regime is also trying to force people to join a pro-Assad demonstration tomorrow. They went to all the schools today and told teachers and parents that their sons and daughters must turn up tomorrow, because they want to take them to this demonstration against their will. That is one reason for the fighting. Another is that this is the people's reply to the Arab League, which was not good for us."

On Sunday the Arab League pledged to consider unspecified action against the regime of President Bashar al-Assad if his bloody crackdown is not replaced by a credible reform programme within 15 days.

A doctor from a hospital in Homs said intimidation had been constant in recent weeks. "The Shabiha [militias] are in here all the time. They are always looking for people. Last week one of the security officers died even though we tried our best to treat him. Then they smashed all our instruments. They have arrested three doctors from this hospital alone."

The global rights organisation Avaaz claims to have gathered evidence of 57 patients being seized from hospitals in Homs and Lattakia and nine doctors detained. It also says it has received seven videos showing ambulances being used to abduct activists and in one case shoot at protesters. It says eyewitnesses have provided it with testimony alleging some protesters have been killed while waiting for treatment. The claims could not be independently verified.

Alice Jay, campaigns director at Avaaz, said: "Not content with killing civilians on the street, the Assad regime has sent its thugs into hospitals to carry out its dirty work, terrorising doctors and murdering injured protesters. The clamour for action to stop this brutality is building from Europe and the US to the Gulf, but one country stands in the way: Russia, which is supplying the Assad regime with arms and blocking tough, targeted sanctions."

Red Crescent workers in Damascus are also claiming they are regularly harassed by military and intelligence officials. "We had to co-ordinate with the security forces to go anywhere," a former Damascus- based Red Crescent volunteer said.

said she and colleagues were twice detained by soldiers and interrogated in recent months.

In Homs, residents are bracing for an intensification of a military campaign, which has seen Syrian forces fighting defectors most days and nights for the past fortnight.

"We know we are facing a bad destiny," said Fadi. "They are planning for something, we know that and we can feel it, but we don't know what it is."