Anti-Gaddafi sentiment is also building in the once quiescent southern city of Sabha, where young men and members of a big anti-government tribe were protesting in the streets and readying their weapons - some brought in from rebel forces in the north - to join the fight.
"The situation is bad, very bad," a rebel spokesman, who gave his name as Ibrahim, said by telephone from Zawiyah. "Fierce fighting is taking place now. The (pro-Gaddafi) brigades have been receiving reinforcements…there are many snipers on roof tops of buildings and mosques. They are the main threat to the residents," he said.
"Sixteen rebels have died in Zawiyah, and we heard that dozens of Gaddafi fighters have been killed," reported Ahmed Haddiya, who heads Misrata's Free Libya radio, and is in contact with fighters there.
Regime troops have kept tight control of Zawiyah after they brutally crushed rebels there in February and March.
On Saturday fighting broke out again, and intensified yesterday.
Regime forces have blocked the road to the Tunisian border purportedly to "prevent the flow of refugees" from the town. The road is Gaddafi's one remaining supply route.
The spreading of fighting to Zawiyah brings increasing isolation to the capital, even raising the possibility of it being cut off.
Yesterday battles were also being fought to the south-west of Tripoli, in the Nefusa mountain towns of Zintan and Yafran, and to the east of the capital at Dafniya near Misrata.
"Many of the rebels fighting in Zawiyah are from the Nefusa mountains," rebel sources said.
Yesterday afternoon a fishing vessel laden with fighters and weapons left Misrata port for Zawiyah.
The massing of tribal fighters in Sabha could kick away another finger of Col Gaddafi's weakening control of Libya. It had until now remained untouched by the revolt that erupted in mid-February. Sabha, an oasis town in the Sahara desert with a population of 100,000 people, is also home to an important military base.
After weeks of battles in a string of towns along the mountains to the south-west, rebels, and Nato airstrikes, have succeeded in controlling much of the area.
Rebels on the frontline at Dafniya say the next objective is to conquer the town of Zlitan - the biggest population centre between Misrata and Tripoli.
Yesterday, under a barrage of Grad rockets and small arms fire, rebels sent forward mechanical diggers to build defences further into enemy territory. "Every day we plan to gain a kilometre like this," said Salah Mabrook, 31, a fighter in the Martyr Brigade.
More than 200 hundred fighters from Zlitan crept behind regime lines to reach Misrata last week, after having received weapons instruction from the Misrata rebels.
"Now they are ready," said Abdul Hadi, 31, a rebel trainer.
A plan last week for the Zlitan fighters to lead an offensive towards their hometown, was set back when regime forces captured one of their commanders.
"Maybe they made him speak, and give away our military strategy," said Ali, a fighter from Zlitan who feared repercussions on his family if he gave his full name. "We have moved the command room, and are re-making the strategy. Then we will attack."
In Tripoli itself, residents have told of anti-Gaddafi protests, though these have been quickly dispersed by his security forces.
"The districts of Tripoli are waiting for a signal so they can all rise up together," said a resident of the city who did not want to be named for fear of reprisals.