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 s heikh 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.
A Foreign Office spokeswoman said: "The UK opposes the death penalty in all circumstances and in every country. The death penalty undermines human dignity and there is no evidence that it works as a deterrent.
"The Foreign Secretary regularly raises human rights issues with his counterparts in countries of concern, including Saudi Arabia. We seek to build strong and mature relationships so that we can be candid with each other about those areas on which we do not agree, including on human rights."
Mr Farron said: "I utterly condemn Saudi Arabia for the execution of 47 people including the prominent Shia cleric Sheikh Nimr al-Nimr. Capital punishment is utterly abhorrent and the Prime Minister needs to turn round to our 'ally' and tell them capital punishment is wrong.
"In 2014, 90 people were beheaded in the kingdom, the highest in two decades. In just two days they have killed 47 people.
"Britain must live our values and criticise nations like Saudi Arabia that continue this heinous and barbarous punishment."
The execution of the sheikh has raised fears that his nephew Ali al-Nimr, who was 17 when he was arrested, could also have the death sentence imposed on him carried out.
Labour leader Jeremy Corbyn used his conference speech in September to call on Mr Cameron to intervene in the case.
Mr Benn told the Press Association: " With the carrying out of this large number of executions there will now be huge international concern about what will happen to Ali Mohammed al-Nimr who is Sheikh al-Nimr's nephew. He was just 17 years old when he was sentenced to death.
"The Foreign Secretary has told Parliament that he does not expect Ali Mohammed al-Nimr to be executed but he now needs to seek fresh assurances that he will be reprieved."
Dozens of protesters gathered outside the Saudi embassy in London to express their anger about the executions.
Amnesty International UK's Shane Enright told the Press Association that the death penalty was "unacceptable in all circumstances" and it was particularly concerning that a number of "peaceful dissidents" had been killed, including sheikh al-Nimr.
Speaking at the scene of the demonstration, he said a recent Amnesty report concluded that the trial against him was "deeply flawed", adding: " We also came to the conclusion that he was jailed solely for expressing his peaceful points of view, protesting peacefully against the regimes.
"This is an absolute, fundamental, breach of basic human rights."