Prime Minister David Cameron has described the tensions provoked by the execution of a prominent Shiite cleric in Saudi Arabia as "hugely concerning".
Mr Cameron said that the UK condemned the death penalty in all circumstances and had made representations to the Saudi authorities on this occasion.
The announcement on Saturday of the execution of 47 alleged terrorists, including Sheikh Nimr al-Nimr, prompted an international outcry, and sparked protests in Shia-dominated Iran, where the Saudi embassy was stormed.
Riyadh responded by announcing it would sever diplomatic ties, with staff from the Iranian mission given 48 hours to leave and Saudi counterparts recalled home from Tehran.
Al-Nimr had been a leading critic of the Saudi authorities and there has been international concern over a sentence of death by crucifixion handed down to his nephew, Ali al-Nimr, for taking part in an anti-government protest in 2012 at the age of 17.
Speaking during a visit to east London, Mr Cameron said: "It is hugely concerning because of course we want to see stability in the Middle East, we want to see good relations between the different countries in the Middle East, not least because that will be absolutely essential for solving the crisis in Syria which is the source of so many of these problems.
"And specifically on Saudi Arabia let me be clear - we condemn and do not support the death penalty in any circumstances and that includes Saudi Arabia."
"We always make representations on the death penalty and the Foreign Office ministers made it very clear on this occasion."
With the threat of further sectarian violence triggered by the executions in predominantly Sunni Saudi Arabia, the UK called for calm.
Foreign Office minister Tobias Ellwood urged all parties in the region to show "restraint and responsibility".
Mr Ellwood said: "I am deeply disturbed by the escalation in tensions in the last 24 hours in the Middle East.
"The UK is firmly opposed to the death penalty. We have stressed this to the Saudi authorities and also expressed our disappointment at the mass executions."
The Middle East minister said he expected that Ali al-Nimr and others who were convicted as juveniles will not be executed, and promised the UK will continue to raise their cases with the Saudi authorities.
A Saudi Justice Ministry spokesman told the BBC: "As a judicial power or judicial branch, we execute, or we apply, Sharia law according to the facts we see in front of us, irrespective of other influences, and that is what the judges are qualified on. They don't look to any other influences, they deal with the facts that they see in front of them."
The justification for Sheikh al-Nimr's death sentence "revolves generally around conducting acts that led to the killing of innocents and confronting security officers and killing them", said the spokesman.
Liberal Democrat MP Tom Brake called for assurances from Mr Cameron that the weapons used in the Saudi executions were not made in the UK.
In a letter to the Prime Minister, Mr Brake said: " The Liberal Democrats and I believe that the executions which we witnessed at the weekend, with some people being executed after they were arrested for instigating political protests, including Nimr al-Nimr, constitute a breach of the most basic human rights laws.
"I am therefore asking if you can give me assurances that no weapons or any other materials made in the United Kingdom were used to prepare or carry out the executions."