The demonstrations follow a new round of government air strikes in the eastern part of Libya. Pro-Moammar Gadhafi forces are centering their raids on oil refineries near Brega.
For the first time, President Barack Obama publicly called on Gadhafi to step down.
"The violence must stop. Moammar Gadhafi has lost legitimacy to lead and he must leave," Obama said on Thursday.
The president even left open the idea of establishing a no-fly zone over Libya. The Pentagon though has tried to play down the possibility of using military force.
Gadhafi has warned that thousands of Libyans will die if the U.S. or NATO intervenes.
On Friday, forces loyal to Gadhafi fired tear gas at protesters in Libya's capital. While Gadhafi tightened his hold on Tripoli, his forces also made an intensified assault on Zawiya, the closest opposition-held city to Tripoli.
An activist said Libyan rebels' military commander in Zawiya, the closest opposition-held city to the capital, was killed along with three other people in fighting. But the activist said Zawiya remained in opposition hands.
Zawiya's fighters have beaten back several attacks in the past two weeks by Gadhafi forces trying to re-capture the city.
So far, the Libyan leader's troops has been unable to take back significant ground from the opposition, which has taken over the entire east of the country and several cities in the west near Tripoli.
The Associated Press contributed to this story.