Human rights activist Salwa Bugaighis has been shot dead by unknown assailants at her home in the restive east Libyan city of Benghazi, hospital and security sources said.

"Unknown hooded men wearing military uniforms attacked Mrs Bugaighis in her home and opened fire on her," said a security official on Wednesday.

She was shot several times and taken to hospital in critical condition, where she died shortly afterwards, a spokesman for the Benghazi medical centre said.

Her husband, who was in the family home at the time of the attack, has since been reported as missing, according to a family member.

"We've lost touch with him," the relative said, adding that a security guard at the house had been shot and injured, but his life was not in danger.

'Shameful act'

Bugaighis, a lawyer, played an active part in Libya's 2011 revolution, which overthrew the regime of Muammar Gaddafi. A former member of the National Transitional Council, the rebellion's political wing, she was vice president of a preparatory committee for national dialogue in Libya.

The US ambassador to Libya, Deborah Jones called the news "heartbreaking", and on her Twitter account denounced "a cowardly, despicable, shameful act against a courageous woman and true Libyan patriot".

Earlier on Wednesday, Bugaighis had participated in Libya's general election. She published photos of herself at a polling station on her Facebook page.

Since the 2011 revolution, the east of Libya -- and in particular the country's second city of Benghazi - has been a stronghold for armed fighters, and the scene of attacks and assassinations targeting notably the military, police and judges.

At least three soldiers deployed to provide polling day security in Benghazi were killed in what security officials said was an attack on their convoy by an armed group.

Benghazi, which was the scene of a deadly attack on the US consulate in 2012, has been tense since former rebel commander Khalifa Haftar launched an offensive late last month, drawing many regular army units to his side.