The Libyan capital may seem peaceful in daylight, but when the sun sets rebels and Gaddafi's forces clash, locals say

In daylight there is the pretence of normality: people chatting in cafes, shops open for business, motorists on the move – if they're lucky enough to still have petrol. But at night, Tripoli takes on a more menacing aspect.

"There are drive-by shootings at night here," said one man in the Souk al-Juma district, an opposition stronghold. "People are shooting at the police every night."

Numerous witnesses tell the same story: that when night falls, out come the police checkpoints aimed at locking down restive districts, but so too do rebel militas opposed to Muammar Gaddafi. Under cover of darkness, it is said, they emerge from hiding to ambush his security forces. In some neighbourhoods the gun battles rage every night, but the bodies of those killed and all other traces are swiftly removed.

With security tight and little sign of a major uprising in Tripoli, these audacious guerrilla tactics appear to be the rebels' best hope of chipping away at the Libyan leader's defences.

In Souk al-Juma there have been reports of explosions, a raid on a police station and at least one public protest violently crushed. A rebel sympathiser, pointing to a street off the district's main road, said: "On one night a couple of weeks ago, four people were killed here."

Libyan government officials denied such attacks have taken place, and the movements of foreign journalists are strictly controlled, making it impossible to verify these claims.

However, anecdotal evidence suggests that there are now regular night-time clashes in the Libyan capital .

The raids have turned parts of Tripoli, a city with no shortage of privately owned firearms, into a no-go area after dark. The man, who said he has stocked up on diesel in readiness for "zero hour", added: "Normally wedding parties go on until 2am, but now they finish at 8pm. No one goes out after sunset. They all stay at home."

Some in his working-class suburb felt it was too dangerous to talk to journalists, employing euphemisms such as "I think you know what I mean." But one young man, sitting in a T-shirt beside a blaring stereo, made an obscene gesture and shouted openly: "Fuck Gaddafi!"

The size and strength of the armed insurgency is unknown. It seems highly unlikely they could raise enough men to topple Gaddafi without external support. Their main strategy appears to be a methodical sapping of the police state's morale.

A man using the name Niz, who claims to represent a Tripoli rebel network known as the Free Generation Movement, said via Skype: "There are armed operations pretty much every night, normally at checkpoints. There is regular gunfire every night. It's not an attempt to go all out. It's an attempt to intimidate the security apparatus and show the presence of an armed group."

Nato hopes that Tripoli would succumb to the Arab spring were swiftly thwarted, with dissidents killed, arrested or driven underground. Apart from occasional public demonstrations, opposition to Gaddafi in the capital remains covert and fragmented, especially since access to the internet and mobile text was blocked. Informants and secret police ensure that fear, suspicion and paranoia are thick in the air.

Niz, speaking English, said he was unwilling to meet in person for fear of arrest. It was therefore impossible to verify his identity, but his account resonates with other media reports.

He said the Free Generation Movement, which posts regularly on Facebook and YouTube, has few members but is in close contact with several similar groups. He insisted that "activity is increasing, fear is decreasing and security is becoming fragmented" in Tripoli, making the ultimate overthrow of Gaddafi inevitable.

"I believe there can be an uprising in the city," he said. "We are moving towards that. I don't believe a single action will bring down the regime, but the coming closer of rebels and the continued Nato campaign shows the noose is tightening. It's going to be a combination of things. The regime will fall."

The resilience of Gaddafi's stronghold over more than four months has already confounded many. Asked to predict when the end will come, Niz replied: "Six million Libyans are asking the same question every day. What I can tell you is that every day there is more and more activity in Tripoli. The pressure on Gaddafi increases as the circle closes. We're all doing our bit to play our part."

But the risks remain high. Plain-clothes police reportedly go from house to house looking for real or perceived rebels. Niz, whose group performs acts of civil disobedience, alleged that people had been kidnapped and tortured by electrocution in increasingly overcrowded prisons.

"I feel uncomfortable using the word 'arrested'; these guys are being kidnapped," he said. "People I know have been blindfolded in the back of trucks and have heard on the radio prison officials saying, 'Don't bring them here, we're full.' There are thousands of people unaccounted for. I've heard people have been tortured, mainly through electrocution and beatings. You also hear about the pulling of fingernails."

A pro-Gaddafi rally on Friday brought tens of thousands of people to Green Square in a formidable show of strength, but Niz estimates that three-quarters of Tripoli's 2 million residents are against the regime.

Moussa Ibrahim, spokesman for the Libyan government, dismissed reports of underground networks in Tripoli. "Dream on," he said. "David Cameron and Nicolas Sarkozy are so embarrassed that they have to make up lies."