Saudi Arabia has been condemned after executing dozens of prisoners, including a cleric who was a prominent figure in the Arab Spring protests against the country's monarchy.
Shadow foreign secretary Hilary Benn said it was "profoundly wrong" to carry out the death sentence against Shiite Muslim cleric Sheikh Nimr al-Nimr, who was among 47 people executed.
International human rights group Reprieve said three other people involved in anti-government protests were among the dead, including two who were teenagers at the time of their arrest, and said David Cameron could not turn a "blind eye" to the executions.
Liberal Democrat leader Tim Farron responded to news of the executions by describing capital punishment as "abhorrent" and called on the Prime Minister to do more to pressure foreign governments into abolishing the death penalty.
Reprieve said sheikh Nimr, Ali al-Ribh, Mohammad Shioukh and Mohammad Suweimal were all arrested in 2012 following their involvement in anti-government protests.
Al-Ribh was just 18 when he was detained, while Shioukh was a year older.
Maya Foa, director of the death penalty team at Reprieve, said Saudi Arabia had executed more than 150 people during 2015, many for non-violent offences.
"Today's appalling news, with nearly 50 executed in a single day, suggests 2016 could be even worse.
"Alarmingly, the Saudi government is continuing to target those who have called for domestic reform in the kingdom, executing at least four of them today. There are now real concerns that those protesters sentenced to death as children could be next in line to face the swordsman's blade.
"Saudi Arabia's allies - including the US and UK - must not turn a blind eye to such atrocities and must urgently appeal to the kingdom to change course."
Mr Benn said: "Saudi Arabia was profoundly wrong to have executed sheikh Nimr al-Nimr. Amnesty International expressed serious concerns about the charges and his trial, and we are opposed to the use of the death penalty in all circumstances."
There are fears that the killing of al-Nimr could spark new unrest among Saudi Arabia's Shiite minority, largely concentrated in the kingdom's east, and in Bahrain, which has seen low-level violence since 2011 protests by its Shiite majority demanding greater rights from its Sunni monarchy.
Al-Nimr had been a vocal critic of Bahrain's Sunni-led monarchy, which harshly suppressed the 2011 Shiite-led protests. Saudi Arabia sent troops to help Bahrain quash the uprising, fearing it would spread.