BENGHAZI, Libya — Growing up in Fairfax as the son of Libyan dissidents, Hesham Mansur was well aware of the horrors of Col. Moammar Gaddafi’s government.

But when the U.S.-born computer networking student registered for classes this semester at George Mason University, he had no idea that he would soon be in Libya, trying to overthrow that government himself.

“One of the Facebook comments was, ‘If you’re so brave, go do it yourself,’ ” Mansur, 27, said, recalling the flurry of Internet messages between Libyans abroad as the uprising gained steam. So on Feb. 28, less than two weeks after the protests began, he packed up medical supplies donated from Libyans in the United States, flew to Cairo and crossed into Libya by land.

He was not alone. For Libyans living in the United States, Germany, Sweden, Britain and elsewhere, the sight of their countrymen rising against Gaddafi’s 41-year rule inspired them to put their own lives on hold and race out to help.

“I felt like I’d done all the Twittering I could do,” said Ahmed Hnesh, 29, a management consultant from Falls Church who is friends with Mansur and arrived here a few days before him.

The returnees number in the hundreds, if not more. Many were dissidents or are the children of dissidents. They came back to provide humanitarian aid, moral support, expertise and front-line muscle.

The rebels’ provisional government includes a number of returnees from abroad, their cosmopolitanism and fluency in European languages helping them build diplomatic relationships. For those who choose to fight, overseas experience tends to be less helpful, though the self-proclaimed rebel army commander, Gen. Khalifa Haftr, spent years living in Fairfax.

Hnesh left the United States without telling his parents. Others living abroad have received tentative blessings from parents torn between fear for their safety and pride at their actions.

“For two weeks I pestered my mum,” said Zackariya Waheishi, 22, a U.K.-born Londoner who was in his last semester at his university when he withdrew to travel to his father’s home city. “Finally she said, ‘Well, it looks like you’re going to go, so go, but don’t be foolish.’ ”

Waheishi brought along an olive-green Urban Spirit jacket and his Timberland boots, and headed to the front line.

“I was provided with a gun there,” he said, adding, “When they see that you’re from the U.K., it’s a morale booster for them.”

The Kalashnikov was the first weapon he had ever handled, and he stood with it on the front line, ready to shoot if Gaddafi’s soldiers approached. “I guess I haven’t tried it yet,” he said. “I hope I won’t have to.”

Not all the new arrivals are young and of fighting age. Ali Tarhouni, 60, a professor of economics at the University of Washington, sent a long e-mail to his students in February, explaining why he could not finish out the semester. Then he got on a plane, leaving his wife and children in Seattle.

“Everyone in my family understood,” said Tarhouni, who was recently named the provisional council’s head of finance, economics and oil, and now spends his days meeting with diplomats and trying to reel in oil revenue for the rebel-held east. “My students are cheering for me.”

Tarhouni had been an activist as a student in Libya, and he said that after he came to the United States in the 1970s, he was sentenced to death in absentia. Returning now was exhilarating, he said. “I thought I’d never see my birth country again,” he said. “I thought I would die away. Many people died and never saw this country.”

Those who come acknowledge the risks. With Gaddafi’s forces advancing toward Benghazi last week and the front line fluid, residents know they could wake any morning to find the city besieged. But once the newcomers arrive, apprehensions about safety tend to fall away as they share the elation of a populace that is willing to sacrifice much to hold on to its new freedoms.

Hnesh, Mansur and other friends from the United States have been staying here, helping to organize humanitarian aid. Four of them sat last week at Mansur’s cousin’s house, drinking Turkish coffee and watching TV coverage of the uprising. Outside, shots rang out.

When Gaddafi forces shelled Benghazi last month, sending residents fleeing, the friends stayed as the alleyways become factories for Molotov cocktails.

“I think being here, your sense of normal just gets skewed,” said Hnesh, who flew home briefly last month before returning. “Right now, normal is hearing guns and grenades flying. And seeing the characters that you do, walking around with major guns, and you go back to work and open your laptop and sit in your cube and you’re going: ‘Am I really here? When yesterday I was walking around taking part in a revolution?’ It doesn’t compare.”

Hnesh is not sure how long he will be able to stay, but Mansur and Waheishi vowed they will remain in Libya until Gaddafi leaves.

“To be honest, me personally, I don’t think I’ll make a difference,” Waheishi said. “But I do believe that many stones make a mountain. There’s strength in numbers. And the more people we have, the higher morale is. And it makes me feel good to know that I am part of that.”

The passion drawing people to this revolution has led to the disappearance of at least one American who is not of Libyan descent. Matthew VanDyke, 31, a Georgetown graduate from Baltimore, arrived in Benghazi on March 6 hoping to make a documentary about the uprising.

On March 12, VanDyke said he wanted to go to the town of Brega to shoot some footage, said Nouri Fonas, the Libyan friend whom VanDyke was staying with in Benghazi.

But on March 14, Brega fell to Gaddafi forces and VanDyke stopped answering his phone. His mother, Sharon VanDyke, said she has asked the State Department for help locating him.

Fonas, a tall man in khaki fatigues who is a writer in civilian life but is now fighting for the rebels, looked down, his lips trembling. “Never came back,” he said of his friend.

“Matthew not only has family in America, he has family in Libya,” he said. “I’d sell my soul to find my friend. He came to help with freedom.”