LONDON (Reuters) - A Saudi activist has begun a hunger strike after being kept in jail past his release date for an eight-year sentence, a human rights group said on Thursday.

Khaled al-Omair had been due to be freed on Oct. 5, after completing a prison term for crimes related to a planned protest against Israeli bombing of Gaza, New York-based Human Rights Watch said in a statement.

He began his hunger strike at al-Hair prison in Riyadh the following day, the statement added, citing Saudi activists.

Another rights group, Amnesty International, reported that he had been tortured and placed in solitary confinement for extended periods while in detention.

Saudi authorities could not be reached for comment on the case. Riyadh denies torture and says its judiciary is independent.

Omair was arrested in 2009 along with 14 other Palestinians and Saudis as they gathered on a street in Riyadh to begin a protest march against the Gaza war.

Most were later released, but Omair and one other man were convicted of "breaking obedience with the ruler" and "embarking on a protest", Human Rights Watch said.

Omair was also convicted of a cybercrime because he called for the protest online.

Before his arrest, Omair had petitioned for democratic reforms in Saudi Arabia, including making a call in 2007 for free elections.

Saudi Arabia prohibits all forms of public protest, saying demonstrations contradict the principles of Islamic law and traditions of Saudi society.

While the kingdom is making gradual moves toward tightly-controlled elections for municipal councils with no lawmaking powers. But the ruling Al Saud dynasty shows no intention of sharing power with elected politicians.

(Reporting by Katie Paul; editing by Andrew Roche)