The armed men arrived this month, pounded on the door and took Ibrahim’s cousin away. There was not a word of explanation and not a word since about where he has been taken.

“I can’t even ask anyone where my cousin is. It’s too dangerous,” the 33-year-old told two reporters who had briefly slipped away from their government minders, on a chance encounter in the mazelike streets of Tripoli’s walled old town.

“Everyone is scared,” he added, looking furtively to the right and left, wary of government informers. “We can only talk to a few close friends. We can’t trust anyone else.”

Human rights groups say the Libyan government embarked on a systematic and widespread campaign to imprison critics in Tripoli after protests against Moammar Gaddafi’s rule erupted — and were violently put down — in February. Ibrahim’s account, and that of other Tripoli residents, suggests that the campaign is continuing this month, albeit at a slower pace.

“Gaddafi and his security forces are brutally suppressing all opposition in Tripoli, including peaceful protests, with lethal force, arbitrary arrests and forced disappearances,” said Sarah Leah Whitson, Middle East and North Africa director at Human Rights Watch. “Given Libya’s record of torture and political killings, we worry deeply about the fate of those taken away.”

The rebel Transitional National Council — the de facto government in eastern Libya — says 20,000 people have been “kidnapped” by the Gaddafi government and are being held in inhumane conditions in several prisons across the capital, as well as in police and army camps and in an old tobacco factory. That figure could not be independently confirmed, but Human Rights Watch said the detentions have been significant and widespread.

The jails include Abu Salim prison, notorious for the massacre of hundreds of prisoners after an uprising in 1996.

Detainees include rebel soldiers captured as the fighting has ebbed and flowed along the coastal towns of Libya. But they also include anti-government activists, journalists, people who organized protests on Facebook or who simply took part in the demonstrations, as well as those who spoke with the foreign media, human rights groups say.

Gaddafi’s powerful second son, Saif al-Islam Gaddafi, conceded in an interview that there had been arrests but he said that no one had been tortured and added that he was supervising a program to release the prisoners.

“It happened because it was a big tsunami here in Libya,” he said. “But the police have started to release them one after another. . . . We are living in the same country. It is not in our advantage to humiliate them, to kill them or to torture them.”

Government spokesman Moussa Ibrahim said he had just one response to the charges: to ask Human Rights Watch and Amnesty International why they had not responded to a government invitation to visit and find out for themselves.

Both organizations said they had not been granted permission to enter government-controlled Libya since the protests and conflict began. Human Rights Watch said the group has received an unofficial spoken invitation to come to Tripoli and is now discussing a formal invitation. Amnesty International says it has received no such offer.

The first wave of arrests followed February’s protests, with government agents even combing the hospitals of Tripoli for people wounded in the demonstrations.

Another wave of detentions took place in other cities as government forces first advanced along the coast in early March and then retreated from the outskirts of the rebel stronghold of Benghazi when NATO airstrikes began March 19. About 1,000 people have disappeared from government-controlled areas in the besieged city of Misurata alone, according to the city council.

Some people have been released after signing a pledge not to repeat offenses “against capacity of the Great Jamahiriya,” Gaddafi’s term for the Libyan state, residents interviewed by The Washington Post and Human Rights Watch said.

But many more are untraceable. Some appear to have been in the wrong place at the wrong time. Ali al-Barg, a 45-year-old doctor and father of four, was last seen tied up next to a military truck and a shot-up ambulance outside the eastern town of Giminis on March 19, lying on the ground with bruises on his face. He was still in his medical scrubs.

He had left Benghazi the night before in a clearly marked ambulance to look for the wounded along the road to Ajdabiya, Human Rights Watch said. His driver was also in custody but a nurse with the group had been shot and killed, according to Hossam al-Majri, a doctor with the Benghazi Medical Committee.

“There were three soldiers there in military uniform and machine guns guarding them,” Nuri Massoud, an ambulance driver who came on the scene told Human Rights Watch. “We tried to talk to them, asking them why they were detaining a doctor, but they ordered us not to talk to them and made us sit down with them for about an hour before telling us to leave.”

The missing also include a number of American citizens, including Reda al-Mizaygri, a Libyan American neurosurgeon from Charleston, W.Va., who was last seen leaving the eastern city of Ajdabiya with cardiologist Idriss Busheri on March 16, heading toward Benghazi in a private car.

American freelance journalist Matthew VanDyke has also been missing since mid-March, according to the Committee to Protect Journalists. A Libyan friend told the family that VanDyke was captured by pro-government forces in the eastern city of Brega and later transported along with more than 1,000 Libyan civilians to the Gaddafi stronghold of Sirte.

In Tripoli, Gaddafi’s supporters cruise the streets in minibuses, waving green flags, or assemble in Green Square by the Mediterranean to chant slogans. Neighborhoods such as Tajoura where the protests were strongest are still encircled by government checkpoints. On Fridays, armed soldiers and militiamen lock down the entire city, while foreign journalists are barred from leaving their hotels.

But still, in whispered asides, Gaddafi’s critics manage to make their views heard. “He is crazy,” one elderly shopkeeper said when his other customers had gone. “Stupid man, he has killed too many of his own people.”

Ibrahim has been forced to close his clothes shop as international sanctions on Libya have cut his supply lines. He still visits the offices of the multinational company where he used to work until its foreign employees fled, but only to check that everything is all right. Mostly, he is reduced to waiting.

“We could build Libya like Abu Dhabi or Dubai, but we have to be free,” he said, echoing a common refrain of many Libyans.

“I have a tricolor in my house,” he added, referring to the black-green-and-red flag adopted by the rebels. “I will bring it out when we are free.”