Amnesty International has expressed alarm at reports that the authorities in Saudi Arabia are planning to execute dozens of people in a single day.
The newspaper Okaz said 55 people were awaiting execution for "terrorist crimes", while a now-deleted report by al-Riyadh said 52 would die soon.
They are thought to include Shia who took part in anti-government protests.
Amnesty said that given the spike in executions this year, it had no option but to take the reports very seriously.
The group believes at least 151 people have been put to death in Saudi Arabia so far this year - the highest recorded figure since 1995.
In 2014, the total number of executions carried out was reported to be 90.
The Saudi newspaper reports said those facing execution in the coming days included "al-Qaeda terrorists" and people from the Awamiya area.
The alleged al-Qaeda militants were accused of attempting to overthrow the government and carry out attacks using small arms, explosives and surface-to-air missiles, Okaz reported.
The Awamiya residents were meanwhile convicted of sedition, attacks on security personnel and interference in neighbouring Bahrain, it said.
Awamiya is a town in the Qatif region of oil-rich Eastern Province.
Since 2011, it has been the centre of protests by Saudi Arabia's Shia minority, which has long complained of marginalisation at the hands of the Sunni monarchy.
Among those at imminent risk of execution were six Shia activists from Awamiya "who were clearly convicted in unfair trials", according to Amnesty.
"It is clear that the Saudi Arabian authorities are using the guise of counter-terrorism to settle political scores," said James Lynch, Amnesty's Middle East and North Africa deputy director.
"Three of those six activists were sentenced for 'crimes' committed while they were children and have said that they were tortured to confess."
On Tuesday, the mothers of five of the six activists wrote to King Salman, imploring him to grant clemency, after learning that their sons had been subjected to a "random" medical examination. They believed it was a sign of impending execution.
Four of the five have been kept in solitary confinement, in a wing housing death row inmates, since being moved to al-Hair prison in Riyadh in early October.
"These executions must not go ahead and Saudi Arabia must lift the veil of secrecy around its death penalty cases, as part of a fundamental overhaul of its criminal justice system," Mr Lynch warned.
Last month, the UK foreign secretary said he did not expect Ali al-Nimr - the nephew of a prominent Shia cleric also sentenced to death - to be executed.
Saudi Arabia argues that death sentences are carried out in line with Sharia and with the strictest fair trial standards and safeguards in place.