Tripoli, Libya (CNN) -- Former rebels attacked the office of Libya's interim prime minister in central Tripoli on Tuesday in an apparent protest over outstanding payments promised by the government to ex-fighters, witnesses said.
The rebels circled the building in heavily armed pickup trucks, carrying weapons including anti-aircraft guns, and opened fire in the air and at the building, an official at the prime minister's office said.
The attackers, some dressed in army fatigues and others in civilian clothing, were from two towns in Libya's Western Mountains, said eyewitnesses and a member of the Tripoli Revolutionary Brigade, another group of former fighters.
Clashes ensued with guards from the prime minister's office, and fighters from the Tripoli Revolutionary Brigade were called in as reinforcements to protect the building, they said.
A member of the Tripoli Revolutionary Brigade told CNN that a fellow brigade member was killed in the clashes.
Roads were sealed off in the area while the situation was brought under control.
The fighters had converged on the building with a number of demands, including the payments promised by the government to former fighters, the official in the prime minister's office and a member of the brigade said.
There have been delays in these payments and similar protests have taken place, including one last month when former rebels opened fire inside the prime minister's building.
While life is slowly returning to normal in the Libyan capital, armed clashes happen occasionally, underscoring one of the main problems facing post-revolution Libya.
Thousands of former rebels are still heavily armed, and the government has not yet been able to disarm them or integrate them into the security forces.
Building a strong army and police force has been one of the priorities highlighted by Libyan officials, but the process has been slow.
This has prompted criticism from many Libyans, who accuse the government and the National Transitional Council of failing to address this issue.
Libya is scheduled to hold elections next month, the first since the fall of Moammar Gadhafi's regime last summer.
Registration of candidates and voters began May 1, ahead of the June 19 vote.