A last-minute appeal to stop the imminent execution of two men in Bahrain has been issued by the UN special rapporteur on extrajudicial killings, Agnès Callamard, as pressure mounts on the country’s king to revoke the death sentences.

Ali Mohamed Hakeem al-Arab and Ahmed Isa Ahmed Isa al-Malali may be executed in the next 24 hours, according to human rights groups.

Callamad said in a statement the men were allegedly tortured, prevented from attending their trial and sentenced to death in absentia.

She added: “The authorities in Bahrain must immediately halt any plans to execute these men, annul the death sentences against them and ensure they are retried in accordance with international law and standards.

“Capital punishment may only be carried out for the most serious crimes and after a legal process which has included all possible safeguards aimed at ensuring a fair trial.

“I remind Bahrain that the only thing that distinguishes capital punishment from an arbitrary execution is full respect for the most stringent due process and fair trial guarantees.”

Amnesty International said it believed the executions were due to take place in the next 24 hours. Lynn Maalouf, the organisation’s Middle East research director, said: “If the Bahraini authorities go through with these executions it would be an utterly shameful show of contempt for human rights. The death penalty is an abhorrent assault on the right to life and the utmost cruel, inhuman and degrading punishment. Its use is appalling in all circumstances, but it is all the more shocking when it is imposed after an unfair trial in which the defendants were tortured to ‘confess’.”

Malali and Arab were convicted in a mass trial involving 60 people in January 2018. Both have exhausted all possible judicial appeals.

Amnesty said: “This afternoon their families were summoned at short notice to visit the men in Jaw prison, where there have also been reports of increased security today, raising alarm that their executions could be imminent. Bahraini law requires that those being executed receive a family visit on the same day that they are killed.”

The small Gulf state, a key US ally located between rivals Saudi Arabia and Iran, has been gripped by bouts of unrest since 2011, when authorities cracked down on Shia-led protests demanding political reform. The UK advises Bahrain on training its police and independent complaints procedures.