A Libyan court has sentenced a son and eight aides of slain dictator Muammar Gaddafi to death for crimes during the 2011 uprising, in a verdict strongly criticised by rights campaigners.

Seif al-Islam, the strongman's one-time heir apparent who is also wanted by the International Criminal Court, was tried in his absence because he is held by militia opposed to the Tripoli authorities.

Former intelligence chief Abdullah Senussi and Gaddafi's last prime minister Al-Baghdadi al-Mahmudi were also sentenced to death on Tuesday, although they can appeal to the supreme court.

The 37 defendants were charged with crimes including murder and complicity in incitement to rape during the 2011 revolution.

Libya has been wracked by conflict since Gaddafi's overthrow, with rival governments and powerful militias battling for control of key cities and the country's oil riches.

The trial, which opened in the Libyan capital in April last year, has been dogged by criticism from human rights watchdogs and an unresolved dispute with the International Criminal Court (ICC) in The Hague over jurisdiction in the case of the Gaddafi son.

Rights groups say the trial was deeply flawed and missed an opportunity to expose the crimes of the Gaddafi regime.

Amnesty International called the death sentences "appalling".

"Instead of helping to establish the truth and ensuring accountability for serious violations during the 2011 armed conflict, this trial exposes the weakness of a criminal justice system which is hanging on by a thread in a war-torn country with no central authority," Amnesty's Philip Luther said.

Human Rights Watch's Joe Stork said the trial was "plagued by persistent, credible allegations of fair trial breaches that warrant independent and impartial judicial review".

The militia holding Seif al-Islam in the southwestern hill town of Zintan is aligned with the internationally recognised government which fled to the remote east last August when a rival militia alliance seized the capital and set up its own administration.

Prosecutor general Siddick al-Sour acknowledged that there was no prospect of Seif al-Islam facing the court's sentence any time soon but said that was a matter for politicians.
"The court pronounced sentence and has nothing to do with the political conflict," Sour said.

Seif al-Islam's sole appearances before the court were by video link and there had been none since May last year.

The UN Security Council referred the conflict in Libya to the ICC in February 2011 amid Gaddafi's repression of the popular uprising against his decades-old regime.

Seif al-Islam is wanted by the Hague-based court on charges of war crimes and crimes against humanity.

ICC prosecutors say that as part of his father's "inner circle", he "conceived and orchestrated a plan to deter and quell, by all means, the civilian demonstrations against Gaddafi's regime".