David Cameron faces growing criticism as he continues to stay silent on the execution of Shia cleric Nimr al-Nimr in Saudi Arabia. Al-Nimr was executed on Saturday for terrorism charges, along with 46 other people.

Speaking on Sky News, leading human right campaigner Peter Tatchell called the Prime Minister's silence "utterly shameful". The veteran activist said: "In British government policy we're against Isis but allied with Saudi Arabia, it doesn't make sense and it's completely immoral."

Shadow Foreign Secretary, Hilary Benn, said that the execution was "profoundly wrong".

Analysts have said the execution could inflame sectarian tensions in the Middle East, with Iran issuing a strong condemnation. The country's supreme leader, Ayatollah Ali Khamenei, said: "The unjustly spilled blood of this oppressed martyr will no doubt soon show its effect and divine vengeance will befall Saudi politicians."

Saudi Arabia's embassy in Tehran was attacked on Saturday during a protest against the execution, demonstrators smashed windows and ransacked the building.

Human Right Watch said that Nimr's conviction had raised "serious fair trial concerns." He was reportedly sentenced to death for "breaking allegiance with the ruler", "inciting sectarian strife" and other charges linked to his role in protests in Shia-majority areas of Saudi Arabia during 2011. Sources in Saudi Arabia have said that Nimr only ever supported peaceful protest.

"Saudi Arabia's harsh treatment of a prominent Shia cleric is only adding to the existing sectarian discord and unrest," said Joe Stork, deputy Middle East director at Human Right Watch.

Maya Foa, director of the death penalty team at international human rights organisation Reprieve said: "2015 saw Saudi Arabia execute over 150 people, many of them 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."

There are now worries for the safety of Nimr's nephew, Ali Mohammed al-Nimr, who has been sentenced to crucifixion for his alleged participation in anti-government protests. When asked about Ali al-Nimr's fate in the House of Commons in October, Foreign Secretary, Philip Hammond stated: "I do not expect Mr Al-Nimr to be executed."

The UK government had to publicly U-turn on a £5.9m contract with the Saudi prison system after coming under huge criticism for the deal.