TRIPOLI, Libya (AP) -- Large anti-regime protests erupted Sunday in several Tripoli neighborhoods where thousands braved the bullets of snipers perched atop high buildings, residents and opposition fighters said. At the same time, hundreds of rebel forces advanced to within 15 miles west of the capital and were rushing forward in pickup trucks and on foot.
Heavy machine gun fire and explosions rang out across many parts of Tripoli on the second day of attacks by rebels from what the opposition called "sleeping cells" inside the city that has been Moammar Gadhafi's stronghold throughout the 6-month-old civil war.
Mukhtar Lahab, a rebel commander closing in on Tripoli and a former captain in Gadhafi's army, said his relatives inside the capital reported mass protests in four neighborhoods known as sympathetic to the opposition: Fashlum, Souk al-Jouma, Tajoura and Janzour. He said mosques there were rallying residents with chants of "Allahu Akbar" or "God is great," broadcast on loudspeakers.
Snipers loyal to Gadhafi on high buildings were firing on protesters in at least one of the four restive neighborhoods, said Lahab. Residents contacted in the city by telephone also reported snipers firing on civilians.
Hundreds of rebels were also advancing rapidly toward Tripoli from the west and the south. Those in the west moved beyond the village of Jedaim to within 15 miles of Tripoli, according to an Associated Press reporter with them at the front.
The AP saw hundreds of rebels at the front line streaming toward the capital, some in pickup trucks and others on foot trying to hitch rides. The mood was euphoric, with some shouting: "We are getting to Tripoli tonight." They were shooting in the air, honking horns and should "Allahu Akbar."
Rebel Murad Dabdoub told The Associated Press that Gadhafi's forces were pounding rebel positions west of the city with rockets, mortars and anti-aircraft fire.
"We are not going back. God willing, this evening we will enter Tripoli," said Issam Wallani, another rebel. He spoke from Jedaim, which has been turned into the staging area for fighters moving toward the capital. He spoke as pickup trucks loaded with fighters headed to the front and the thud of mortar shells was heard at two-minute intervals.
Libyan rebels said Saturday that they had launched their first attack on Tripoli in coordination with NATO and gunbattles and mortar rounds rocked the city. NATO aircraft also made heavy bombing runs after nightfall, with loud explosions booming across the city.
A Tripoli resident said the capital was virtually deserted on Sunday, with stores shuttered and no cars or pedestrians out on the streets. Some areas suffered power outages, according to the resident, who was reached by telephone and would only identify himself by one name, al-Tarhouni.
An AP reporter in Tripoli, meanwhile, said many of the staff at the Tripoli hotel where foreign journalists are staying did not show up for work on Sunday, a development that suggests residents were too frightened to venture out.
"There are thousands and thousands of soldiers who are willing to defend the city," Libyan government spokesman Moussa Ibrahim told a news conference in Tripoli. He accused the rebels of committing atrocities in areas under their control and appealed for a cease-fire.
He warned of "disasters" if Gadhafi's regime falls.
NATO said the shifting battle lines and concentration of fighting in towns and villages are making it more difficult to identify and engage targets for airstrikes.
"It's much tougher to do in an urban area," he said. "This requires very precise and deep intelligence to achieve without endangering the civilian population."
In Dubai, Libya's new rebel-allied ambassador to the United Arab Emirates, urged for stepped up NATO air attacks over Tripoli, including the use of helicopter gunships.
"We are asking for more Apache action" to counter Gadhafi forces clashing with rebels, said Aref Ali Nayed, who is also spokesman for a rebel transition team.
The United Arab Emirates is among the Arab states that have strongly backed the rebellion against Gadhafi and could provide critical assistance if the Libyan leader is ousted.