Although the figures remain murky, human rights workers said "huge" numbers of people were seized by secret police. One activist, writing on his Facebook page, said 68 people were apprehended in Homs alone.

The mass detainment preceded military deployment on Monday morning in the southern town of Deraa, where protests demanding the ouster of Syrian President Bashar al-Assad erupted last month. Syrian officials also closed all land crossings with Jordan, the news agency Reuters reported. 

The crackdown follows a sharp escalation in unrest over the weekend. Syrian authorities have gunned down more than 120 people since Friday, marking the highest concentration of violence since the onset of demonstrations. Rights groups say more than 350 people have been killed in total.

On Saturday security forces fired on mass funerals held across the country as Syrians mourned those killed during Friday demonstrations.

Twelve people were reportedly shot dead during the processions, including some held in the Damascus suburbs of Douma and Barzeh. Demonstrators there chanted “Where are you Assad? We’re after your head.”

Security forces also opened fire in the village of Ezra, in Deraa province, claiming the lives of more Syrian mourners.

The British government urged its citizens to leave the country “while commercial airlines are still flying,” saying there had been a “rapid deterioration in the security situation.”

The British Council in Damascus held a crisis meeting on Sunday night to decide whether to evacuate its British staff this week.

“To be honest, if they don’t close then I will probably decide to leave anyway because the situation is so dangerous,” said one employee who refused to be named. “Most of the other Brits I know are also planning to leave.

“I was in Beirut this weekend and there are a lot of Damascus-based Brits waiting there to see what happens. If it calms down by Wednesday then they will come back. But if it gets worse then they will all leave.”

A journalist living near Damascus, who asked not to be named for fear of reprisal, said wounded protesters are often too afraid to seek medical attention because of reports that people were being arrested on their hospital beds.

Human rights activists confirmed the story. “The security services have been taking people from hospitals,” said President of the Human Right Association of Syria Haitham Maleh.

Syria is one of the most tightly controlled countries in the region, with the widely-feared mukhabarat, or secret police, administering much of the suppression. An Amnesty International 2010 report said torture was commonplace in police stations, detention centers and prisons.

Meanwhile, the military reportedly set up tight cordons around a number of flashpoint cities throughout the country.

Radwan Ziadeh, director of the Damascus Center for Human Rights Studies and a visiting scholar at George Washington University, said the army now manned checkpoints in all of Syria's major population centers.

“Anybody who has suspicious contacts on their mobile phones is taken away," Ziadeh said.

In response to this weekend’s violent crackdown, Amnesty International’s Middle East and North Africa director Malcolm Smart said: “The Syrian authorities have again responded to peaceful calls for change with bullets and batons. They must immediately halt their attacks on peaceful protesters and instead allow Syrians to gather freely as international law demands.”

British Foreign Secretary William Hague said he was “appalled” by the deaths this weekend.

“The Syrian authorities must act quickly and decisively to calm this dangerous situation and can only do so by responding to the legitimate demands of the Syrian people,” Hague added.

In a further development two members of parliament from Deraa resigned on Saturday in protest over the killing of demonstrators. Although the move is largely symbolic, it would have been unthinkable only a few weeks ago.

The resignations coincide with more evidence of the potential unraveling of the regime and its grasp over the security and military apparatuses. Ziadeh, speaking with TIME magazine on Sunday, said low-ranking military officers have begun defecting.

"They're not following orders. The regime knows who it can rely on: the 4th and the Presidential Guards. We hope that the military will play a role," he said. "But if senior politicians don't resign, it won't encourage military commanders to do the same."

There are more than a dozen Syrian security agencies that, thus far, have shown little indication of abandoning the regime. Assad, like his political predecessor and father Hafez, has appointed members of his minority Shia-offshoot Alawite sect as top army brass. Members of the Assad family head the most important security branches. Analysts consider the fate of these officers closely linked to Assad’s survival.

Despite the Syrian opposition’s growing momentum, the Deraa deployment demonstrates the steadfast resolve of the regime to crush dissent with brute force.

Through its media outlets the government has blamed "armed gangs" and Islamist radicals for the violence, saying they are intent on stoking sectarian strife.