Hundreds of Syrian troops stormed the Damascus suburb of Saqba overnight - breaking into houses and arresting about 300 people, witnesses say.
Tanks and troops are also reported to have been sent to other trouble spots, amid fresh reports of anti-government demonstrations in Homs and Hama.
The moves come despite appeals from the UN and US for President Bashar al-Assad to end the violence against protesters.
Activists, meanwhile, have vowed to stage a "Day of Defiance" on Friday.
More than 500 Syrians are thought to have been killed since the protests started seven weeks ago.
At least 2,500 others have been detained, although rights groups say the figure could be much higher.
Foreign journalists are not allowed to enter the country, so it is difficult to verify the reports.
The protests, which began on 15 March, pose the most serious challenge to four decades of rule by the Assad family in one of the Arab world's most repressive countries.
The authorities seem to be going all out to prevent another big protest after Friday prayers, says the BBC's Jim Muir, reporting from the Lebanese capital, Beirut.
Here are some other reports from activists and human rights groups in Syria about the escalating government crackdown:
- In the Damascus suburb of Douma, there were reports that government-backed "thugs" and security forces had raided two hospitals - Hamdan and Noor. They allegedly destroyed medical equipment and beat some doctors and patients before arresting about 10 people
- The suburb of Deraya, meanwhile, was reportedly still under siege, but some 300 protesters defied the security forces to stage a brief rally calling for freedom
- At least 30 people were arrested in the Damascus district of Tal on Wednesday, including some men in their 70s or 80s. Most are members of the same family
- Elsewhere in the country, troops and tanks are reported to have been sent to the town of Rastan near the central city of Homs
- A resident of Baniyas, on the coast, has told BBC Arabic that security forces are massing in the northern neighbourhood of Qosoor. He says the authorities are using pictures of recent protests to go around the city arresting people
Deraa 'mission accomplished'
Deraa, the southern city where the protests began in March, has been under siege for about 10 days but army units have begun a "gradual withdrawal", a Syrian military official was quoted as saying on state news agency Sana.
The military official said the army had completed its mission to "chase elements of terrorist groups... and to restore security, peace and stability".
Since the weekend, almost 1,000 men have reportedly been rounded up in Deraa alone.
Rights groups say that security officials conduct house-to-house raids, carrying lists of wanted people. If they do not find those listed, they arrest relatives instead.
The government's position is that the demonstrators are militant criminals and not ordinary civilians calling for political reform.
A Syrian blogger who lives in Canada has told the BBC that Deraa is a "special case" and that protests in other parts of the country have been relatively small - drawing less than 1,000 people.
Camille Otrakji says the majority of Syrians want serious change, but not regime change, and that President Assad himself is still very popular with large segments of the population.
"Even those who want regime change know that it is not doable [because] we're going to end up with civil war or fundamentalism," Mr Otrakji told the BBC World Service by phone from Montreal.
But the government's use of violence - including live fire - against peaceful protesters has drawn international criticism.
On Wednesday, UN Secretary General Ban Ki-moon called on President Assad to end the clampdown on opposition supporters.
The US has denounced the crackdown as "barbaric" and tightened its sanctions against senior Syrian officials.
Speaking during a visit to Rome on Thursday, Secretary of State Hillary Clinton said that Europe and the US had to work together in the interests of the Syrian people.
"Together we have to show the Syrian government that there are consequences for this brutal crackdown that has been imposed on the Syrian people."