Saudi Arabia faces new calls to release the dozens of people detained since September in what has been condemned as a “worrying” clampdown on human rights.

More than 60 prominent clerics, writers, academics, religious figures, journalists and activists are reported to have been detained in the kingdom in a wave of arrests that began in September.

In a statement on the second anniversary of Saudi Arabia’s last mass execution, of 47 adults, UN human rights experts and groups including Amnesty International and Human Rights Watch have decried the latest crackdown on dissent, which began after Prince Mohammed bin Salman became the new crown prince in June.

In a statement, five UN experts condemned a “worrying pattern of widespread and systematic arbitrary arrests and detention” through the Gulf kingdom’s use of counter-terrorism and security laws.

“We are witnessing the persecution of human rights defenders for peacefully exercising their rights to freedom of expression, assembly, association and belief, as well as in retaliation for their work,” the experts said.

“The government has ignored repeated calls by UN experts and others to halt these violations, rectify them, and prevent their recurrence.”

A prominent preacher, Salman al-Awdah, is reported to be among those held since September; he is described by the UN as a reformist and influential religious figure who has urged greater respect for human rights within sharia.

The UN experts also named as detainees Abdullah al-Maliki, an academic and writer known for his support for reforms and human rights; entrepreneur Essam al-Zamel, who supports economic reforms; and Abdulaziz al-Shubaily and Issa bin Hamid al-Hamid of the banned Saudi Civil and Political Rights Association.

The arrests since September add to previous cases already raised by UN experts with the Saudi government. “We have written to the government requesting detailed information about these numerous arrests on terrorism, cyber-crime or any other state security-related charges during that period,” the statement said.

“Despite being elected as member of the [UN] Human Rights Council at the end of 2016, Saudi Arabia has continued its practice of silencing, arbitrarily arresting, detaining and persecuting human rights defenders and critics,” the experts said.

The five UN investigators have global mandates on human rights defenders, arbitrary detention, the right to freedom of opinion or expression, freedom of religious belief and protection of human rights while countering terrorism.

In a separate statement issued on Tuesday, which highlighted the anniversary of Saudi’s mass execution two years ago, the human rights organisation Reprieve warned of fresh repression under the new Saudi crown prince.

The group raised concerns for 14 political prisoners, including one arrested as a juvenile, who face imminent execution, after their death sentences were upheld in July 2017.

Maya Foa, director of Reprieve, said: “Two years on from a mass execution that saw political protesters – including some arrested as children – killed, the government of Saudi Arabia shows no interest in halting a brutal wave of repression. Hundreds of people have been executed in the last two years, and now several young protesters face imminent execution on Mohammed bin Salman’s watch. The international community – including Theresa May, who is soon to host the new crown prince – must hold him to his promises of ‘reform’ by demanding a halt to all executions immediately.”

Research by Reprieve has shown that several smaller, serial executions were held in Saudi Arabia last year. About 70% of the 141 executions carried out in 2017 happened after Mohammed bin Salman took over in June, it said.

The UN expert statement did not mention the arrests of some 200 princes, ministers and business leaders in November who were held in the luxurious Ritz-Carlton hotel in what Riyadh said was a crackdown on corruption. Some have since been freed after they reached financial settlements with the government.