(CNN) -- Sanaa residents reported hearing heavy explosions and sporadic gunfire for several hours early Tuesday, one day after security forces set fire to tents and tore through demonstrators' camps in Freedom Square in Taiz with bulldozers, an activist and eyewitnesses said.
It was not immediately clear what caused the blast sounds in Sanaa, which residents described as coming from the Hasabah neighborhood, an area close to it and the region near the airport. Hasabah is home to Sheikh Sadeq al-Ahmar, leader of the powerful al-Hashid tribe, whose forces oppose the government.
Eyewitnesses in the capital city said they saw a heavy security presence in Sanaa, as well as what they described as "armed thugs" roaming the streets.
In Taiz, a center of protests against the Yemeni president, the protest camp was essentially gone on Monday, said Bushra Maktati, a leading human rights activist. A field hospital was also dismantled, with the equipment taken away by troops, Maktati said.
Troops also used water cannons to disperse thousands of protesters in the city on Monday, a day after clashes left at least 20 people dead and 200 wounded, according to eyewitnesses and two medical officials who could not be named because of security concerns.
One youth activist said the attacks would not stop their protests.
"Our revolution will not stop even if hundreds are killed every day," said Sameer Al-Samaee, a leading youth activist in Taiz. "Killing innocent civilians always leads to war crime charges and that is what we are seeking for Saleh."
Meanwhile, government forces launched airstrikes against Islamic militants in the coastal city of Zinjibar, where fierce fighting raged Sunday.
And the nation's largest cell phone network was ordered shut down Sunday, according to a senior official with the country's Communications Ministry who requested anonymity because he was not authorized to speak to the media.
The SABAFON network was ordered shut down because of violations and unpaid fines over the last few years, the Communications Ministry official told CNN.
A management official with the SABAFON network, who also was not authorized to speak to the press, confirmed the shutdown. The official denied the government's allegations and said the move appeared to be a tactic to pressure members of the al-Ahmar family, including Hamid al-Ahmar -- President Ali Abdullah Saleh's chief political enemy.
The official said members of the al-Ahmar family are majority shareholders in SABAFON, with the largest shareholder being Hamid al-Ahmar.
Saleh has been under intense pressure to resign after months of protests and mounting opposition.
Taiz, where protests continued Monday, has been a center of anti-Saleh activity. The most recent protests broke out Sunday when thousands of protesters took to the streets and were met with gunfire from security forces.
Protesters threw rocks at the forces, who responded with more gunfire, witnesses said.
The U.S. Embassy in Sanaa condemned what it called the "unprovoked and unjustified attack" on demonstrators in Taiz. It praised the protesters and called on Saleh "to move immediately on his commitment to transfer power."
On Monday, security forces were arresting youths and taking them from the streets to an unknown location, Maktati, the human rights activist, said.
At least 70 tents had been burned down by security forces since late Sunday night, according to witnesses.
Abdu Ganadi, a government spokesman, said security forces were rescuing colleagues who had been captured and beaten by protesters.
"We did not attack the protesters," Ganadi said. "Reports are all exaggerated. Only two were killed."
He said protesters' tents were burned by people attacked by the protesters, and that tents that burned were empty.
In Zinjibar, fighting continued Monday between Yemeni troops and Islamic militants.
Militants moved into the city on Friday and controlled the streets by Saturday, residents said.
The militants began ferocious attacks on Saturday, according to a Yemeni government official who asked not to be identified because he was not authorized to speak to the media. Security forces and soldiers abandoned their posts, leading to chaos, the source said.
Hundreds of soldiers moved back into the city on Sunday, with heavy fighting between militants and the Army's 25th Mechanized Brigade, the source said.
More than two dozen soldiers had been killed since the start of the battle on Saturday, said a government source Sunday who asked not to be named because he was not authorized to speak to the media.
The fighting occurred on the same day Saleh met with top military and security officials to talk about "hostilities and crimes" occurring in his country, the Yemeni State News Agency reported.
Saleh has been resisting protests calling on him to step down after 33 years in power.
The powerful al-Hashid tribe, which includes the al-Ahmar family, rose up against long-time leader Saleh in the last week, after he backed out of a regionally brokered deal meant to ease him out of office and end months of demonstrations of the kind that have swept the Arab world this year.
The recent fighting has raised fears of a full-blown civil war in Yemen, an impoverished, arid and mountainous nation that has been a key U.S. ally in the battle against the al Qaeda terrorist network.