Saudi Arabia reportedly plans to execute more than 50 people convicted of terrorism in what appears to be a warning to would-be jihadists.
Saudi newspaper Okaz claims fifty-five people are awaiting execution for "terrorist crimes" that killed more than 100 civilians and 71 security personnel without specifying when the executions would occur.
Some of those facing execution were affiliated with al-Qaeda, the newspaper said.
The alleged militants stand accused of attempts to overthrow the government and carry out attacks using small weapons, explosives and surface-to-air missiles.
One prisoner is accused of trying to buy nuclear material in Yemen worth $1.5 million for use inside Saudi Arabia.
Others are from Awamiya, a largely Shia town in the oil-producing Eastern Province where the government has suppressed demonstrations for equal rights.
The charges against the six Awamiya residents include sedition, attacks on security officials and interference in neighbouring Bahrain, which has also experienced unrest since 2011.
Three juvenille offenders - Ali al-Nimr - whose case sparked a global outcry earlier this year - Abdullah al-Zaher and Hussein al-Marhoon - are also said to be among those waiting to be executed.
Ali al-Nimr was only 17 when he was arrested for taking part in pro-democracy protests and was 18-years-old when he was found guilty and sentenced to death.
Saudi authorities said he had broken his loyalty to the country's king, Salman.
But Ali's supporters say he was forced to sign a confession after days of torture in custody.
The case has drawn international condemnation, and was mentioned by Jeremy Corbyn at his first party conference speech where he urged David Cameron to get involved.
Amnesty International has expressed alarm over the reports which claim the slaughter will be carried out in a single day.
James Lynch, Amnesty's Middle East and North Africa deputy director told the BBC: "It is clear that the Saudi Arabian authorities are using the guise of counter-terrorism to settle political scores."
The human rights group believes at least 151 people have been put to death in Saudi Arabia so far this year - the highest recorded figure since 1995 - and said it takes the latest reports seriously.
The last time Saudi Arabia carried out mass executions for security offences was after a group of Islamist militants seized Mecca's Grand Mosque in 1979.
Local residents have responded to the news by closing off roads leading into the city with burning debris in protest.
By Katie Paul, Angus McDowell
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