Syria has sent more troops to the southern town of Deraa where activists say a deadly crackdown against pro-democracy protesters has entered a second day.
The developments in the volatile city, near the border with Jordan, come despite mounting international pressure on Bashar al-Assad's government to end the violence.
Security forces have shot dead 400 civilians since mid-March, Sawasiah, a Syrian human rights organisation, said on Tuesday. Up to 500 people had been arrested in the last two days, it said.
"We've been listening to live ammunition. Some snipers are working as well, but we don't know from where," a resident of Deraa told Al Jazeera on Tuesday.
"The snipers are on all the roofs. I'm now on my stomach, on the ground - I am really in a panicked situation. The city is quite in danger."
Al Jazeera's Rula Amin, reporting from Damascus, said the government has reiterated that the troops are in Deraa to protect residents.
"What we are hearing from activists in Damascus is fear and concern that what the government is trying to do is crush the protests to create fear among people to stay at home," she said.
"Then [the government will] come up with its own plan of reforms, but people won't be able to stand up and defy these reforms. That is how the government wants to move forward."
Last week Assad lifted Syria's 48-year state of emergency and abolished a hated state security court. But the next day 100 people were killed during protests across the country.
The Syrian government insists the army was invited in to rid Deraa of armed men.
But witnesses said soldiers began opening fire on civilians indiscriminately after arriving, sparking panic in the streets.
Abdullah Abazid, an activist in Deraa, told the AFP news agency: "New army troops and security reinforcements have entered Deraa and a tank was deployed in the central Kaziat al-Balad square."
He said troops were firing on residents and a mosque and had laid siege to the home of Deraa's chief religious leader, Mufti Rizk Abdulrahman Abazid, who quit last week in protest at the crackdown.
"The bullets continue against the people, but we are resisting," he said.
Abdullah Abazid said the shooting continued well into the afternoon. The Abu Bakr Assidiq mosque, schools and reservoirs were hit and shops looted, he said.
He said "tanks and roadblocks are deployed at the entrance to the town", preventing anyone from entering.
Residents complained that water and electricity had been cut throughout Deraa since dawn on Monday, when 3,000-5,000 government troops backed by tanks and snipers rolled in.
Verifying information on the military operation in Deraa has become extremely difficult after what appears to be the deliberate jamming of mobile phone signals across large parts of the city and the cutting of landlines and internet access.
Neither Syrian nor Jordanian mobile phone numbers have been reachable in most parts of Deraa since Monday.
Sawasiah, the Syrian rights group, said on Tuesday it had received reports that at least 20 people had been killed in Deraa.
"Witnesses managed to tell us that at least 20 civilians have been killed in Deraa, but we do not have their names and we cannot verify," a Sawasiah official told Reuters.
Douma and Baniyas crackdown
Sawasiah said that two more civilians were confirmed dead after government forces entered Douma, a suburb of the capital, Damascus.
A huge crackdown began on Monday in Douma and nearby Maadamiye, according to activists and residents reached by telephone.
By Tuesday afternoon Douma had become a "ghost town and the presence of security forces decreased", one resident told the AFP news agency.
A witness said three doctors were arrested from Douma's Hamdan hospital and security forces made patients leave the facility, even those in intensive care.
A Deraa resident describes to Al Jazeera a desperate situation in the besieged southern Syrian city
Another massive demonstration took place on Tuesday afternoon in Baniyas, an oil-refining port on Syria's Mediterranean coast, in defiance of a reported military build-up around the city.
A protest organiser said up to 5,000 people began the demonstration at about 5pm, which quickly grew to an estimated 10,000, chanting "With our soul and blood we sacrifice to Daraa" and "The people want to topple the regime."
In a riposte to government claims that the protests are sectarian in nature, the demonstrators chanted: "Syrian people are one, no sectarian division."
"This is not a sectarian movement," said the protest organiser. "I know officers in the army from different sects who refuse to shoot at protesters and are being executed. I have spoken to soldiers who told me that they will not open fire and that they will run away when the time is right."
A second protest leader said security forces had deployed on Tuesday in the hills around Baniyas in preparation for a possible attack to crush the popular uprising there.
"Forces wearing black and carrying AK-47s deployed today in the hills. Armoured personnel carriers passed by the highway adjacent to Baniyas at night," Anas al-Shaghri told Reuters from the city.
"We are expecting an attack any moment. We will receive them at the gates with our bare chests."
The first protest leader told Al Jazeera he had taken count of between 35 to 40 tanks stationed around Baniyas.
"I don't think the army will enter Baniyas like they did in Deraa because there is an oil refinery which they could hit," he said. For activists like himself and al-Shaghri, he said, the effort to bring down the Syrian government had become a zero-sum game.
"Before we knew that when they arrested people they would put them in prison. Now when they arrest people, we never see them again." he said.
In addition to blocking phone signals, authorities referred Mahmud Issa, a prominent dissident, to military court for owning a satellite phone, a week after his arrest and an interview he gave to Al Jazeera, according to a leading activist.
"Mahmud Issa will be tried by a military court for owning a Thuraya phone and a high-tech computer," Rami Abdel Rahman of the Syrian Observatory for Human Rights, a Syrian rights group, said.
Issa was twice jailed in Syria for a total of 11 years.
Abdel Rahman said another activist, Qassem Azzawi, was arrested on Tuesday for taking part in an anti-government protest last week, and that 43 people were rounded up since Monday in raids across the country while 11 others were freed on Tuesday.
Wissam Tarif, director of Insan, a human rights organisation, told Al Jazeera on Tuesday that his website had been left hacked into and more than 2,000 spam messages left on it, effectively crippling the site.
He said his Facebook and Twitter accounts had also been hacked into and hundreds of threatening and abusive messages left.
"The messages said that I am history, that they will kill me and they want to drink my blood," he said. "They also seem to have a big problem with my mother and other members of my family."
Hacking into activists' social media accounts using "phishing" operations has been a tactic other governments in the region have used to try to prevent news of uprisings in their own countries from reaching the outside world.