There were protests in Benghazi at the killing of Abdelsalam al-Mismari
A prominent Libyan political activist and two senior security officials have been shot dead in the restive eastern city of Benghazi.
Activist Abdelsalam al-Mismari was killed as he left a mosque after Friday prayers, officials said.
An air force colonel and a senior police officer were also killed in separate attacks.
Libya's government is struggling to control armed groups nearly two years after Muammar Gaddafi was toppled.
The BBC's Rana Jawad in the capital Tripoli says that although Benghazi has witnessed many targeted killings, Friday saw the first assassination of an activist.
Mismari, a lawyer, was one of the earliest organisers of protests that eventually led to the overthrow of dictator Col Muammar Gaddafi.
He later became a critic of the armed groups that helped to topple Gaddafi but which have since refused to lay down their weapons.
He has also opposed the presence of the Muslim Brotherhood in Libya.
"He was coming out of Friday prayers when he was shot," said Benghazi security spokesman Mohammed al-Hijazy.
"It seems it may have been the work of a sniper because he was shot in the heart."
His colleague and friend Hannah Ghallal told the BBC Mismari was "a hero and a man of principle who did what he preached". Fighting back tears, she added that his death was a loss for Libya.
A large crowd later gathered in central Benghazi to protest against the killing.
Our correspondent says Mismari's death marks a potentially dangerous turning point. Some feel it is an attempt to silence civic groups, she adds.
In separate attacks on Friday, retired air force Colonel Salem al-Sarah was killed as he emerged from a mosque and police Colonel Khatab Abdelrahim al-Zwei was shot dead at the wheel of his car, officials said.
Benghazi has seen a number of violent incidents since the fall of Gaddafi, including an attack on the US consulate last September in which the US ambassador and three other US citizens were killed.