* More urban resistance could lend new dimension to conflict
* Hundreds jailed in capital, suspected of rebel sympathies
* Government denies any rise in rebel activity in Tripoli
TRIPOLI, April 11 (Reuters) - Overwhelmed by the superior firepower of Muammar Gaddafi's troops, opposition fighters in western Libya are resorting increasingly to guerrilla tactics in their campaign to topple the veteran leader.
Unlike eastern Libya, where rebels hold many coastal cities, the west of the country remains firmly under Gaddafi's control.
The proximity to the nerve centre of Gaddafi's powerful military apparatus in the capital Tripoli has made it hard for fragmented dissenters to organise their actions into a movement.
But that may now be changing. Tripoli residents said there had been several attacks on army checkpoints and a police station in the past week, and gunfights can be heard at night.
In one attack a week ago, opposition supporters stormed a checkpoint in eastern Tripoli and seized arms, residents said.
"There have been attacks by Tripoli people and a lot of people have been killed on the Gaddafi army side," said a Libyan rebel sympathiser who lives in exile abroad and maintains daily contact with colleagues in the restive suburb of Tajoura.
Asked who the attackers were, he said they were local residents who wanted to topple the Libyan leader.
Either part of a broader rebel plan or simply a spontaneous evolution of tactics, the shift towards more urban resistance could add a new dimension to the two-month-old war and work to erode Gaddafi's support base in his main western stronghold.
Another resident said that in places like Tajoura, the government controlled only key junctions and roads, where it has checkpoints reinforced with anti-aircraft guns and tanks. But smaller streets deep inside suburbs were outside their control.
These reports could not be verified independently. Information is difficult to piece together because the government does not allow journalists to report freely in the capital. Suburbs such as Tajoura are off limits to reporters.
Residents have told Reuters there have been more gestures of defiance in the past week, including a street protest in the neighbourhood of Fashloom -- a rarity in Tripoli since a fierce crackdown on anti-Gaddafi demonstrations in early March.
An opposition Facebook group has posted a video of what it described as a protest on April 7 in Fashloom, a working-class suburb and the site of earlier clashes.
In the video, a group of men, their faces hidden by scarves, hold anti-Gaddafi banners and one of them reads out a statement declaring his allegiance to rebels.
"We are demonstrating yet again after we sacrificed hundreds of martyrs," he said.
Residents said security agents were using a new tactic by posing as protest organisers to lure dissidents into the streets.
Describing an incident in Fashloom three days ago, a Tripoli resident said: "It was difficult to understand what was going on, but it later came to light that some undercover security men were posing as activists and protesters.
"When people came out into the streets to join in, they were immediately arrested. Most have not been released, nor has information been given as to their whereabouts."
The Libyan man in exile said that in an incident on April 8 two cars waving the tricolour flag of the anti-Gaddafi movement appeared in another Tripoli suburb. "It was a trick. These cars belonged to Gaddafi gunmen. When protesters gathered around these cars they started to shoot at civilians," he said.
The government denies using force against civilians and says people are free to hold peaceful protests and express their views. It denies any rise in underground rebel activity.
"We have heard of these reports. They are false," said government spokesman Mussa Ibrahim.
"They are talking about shootings between rebels and the government. This has not happened. Tripoli has been peaceful. ... There is no organised rebellion."
He also denied that government agents were posing as rebels to provoke dissenters.
Emboldened by rebel successes in the east, several western cities tried to rise up against Gaddafi in March but revolts in places like Zawiyah and Sabratha have been suppressed violently.
Militiamen have used live ammunition against protests in Tripoli, locals say, and hundreds of young men have been jailed on suspicion of being rebel sympathisers.
At night, a network of military checkpoints springs up around Tripoli and militiamen stop all passing traffic although there is no official night-time curfew.
Gunfire regularly rings out in Tripoli at night. The country is awash with weapons and people often shoot in the air as a celebration. But residents have been alarmed by what sound like gunfights in the dead of night.
"Many believe there have been small sporadic battles being fought in some of Tripoli's districts like Souk al-Juma," a witness was quoted as saying by the BBC on April 8.
The witness reported hearing what sounded like explosions and said he had heard that a police station in the opposition-minded Souk al-Juma suburb had been raided.
By day, Tripoli is in a security lockdown and there are no outright signs of protest or dissent.
Patriotic songs blare on street corners and cars plastered with portraits of Gaddafi speed around sounding their horns. People are reluctant to voice their opinions.
Social networks have been flooded with contradictory rumours about rebel attacks in Tripoli, and residents have given conflicting accounts of what they hear and see at night. Some users have suggested that rebels had infiltrated the city.
The picture is similar in other parts of western Libya.
In Zlitan, people said the security crackdown was increasingly tough in their small dusty town just west of the besieged city of Misrata, where rebels are fighting Gaddafi troops in an increasingly violent standoff.
"No one wants him any more. He has to go," one rebel sympathiser in Zlitan told Reuters. He said fighters from Misrata brought their wounded to Zlitan for medical treatment but that was hard because of an intensifying security crackdown.
Silhouettes of what looked like gunmen could be seen on the rooftops on a recent visit to Zlitan. Its streets were almost deserted of civilians and many shops were boarded up.
Pointing at a large concrete building in central Zlitan, the man added: "They have a close watch on all our movements. That whole building is packed with intelligence. They are trying to divide us."