Former Bangladesh prime minister Sheikh Hasina and her government tried to cling on to power using systematic, deadly violence against protesters that could amount to “crimes against humanity”, the UN has said.

UN human rights investigators accused the deposed government of a brutal response to mass opposition last year, in which they said up to 1,400 people had been killed, mostly by security forces.

The UN team said “an official policy to attack and violently repress anti-government protesters” had been directed by political leaders and senior security officials.

Hasina, who had been in office for 15 years, fled by helicopter to India shortly before crowds stormed her residence last August.

The unrest began as student-led protests against quotas in civil service jobs and escalated into a countrywide movement to oust Hasina and her Awami League party following a deadly police crackdown. Thousands more were injured in the worst violence Bangladesh has seen since its war of independence in 1971.

The UN investigators’ findings show the then government, including Sheikh Hasina, “were aware of and involved in very serious offences”, UN human rights chief Volker Türk told a news conference in Geneva.

“Among our key findings, there are reasonable grounds to believe that officials of the former government, its security and intelligence apparatus, together with violent elements associated with the former ruling party, committed serious and systematic human rights violations,” Mr Türk said.

The UN investigators documented the shooting at point-blank range of some protesters, the deliberate maiming of others, arbitrary arrests and torture.

Children, too, were targeted – the report estimates up to 13% of the 1,400 people killed between between 1 July and 15 August were children.

“The brutal response was a calculated and well-co-ordinated strategy by the former government to hold onto power in the face of mass opposition,” Mr Türk said.

He said the evidence gathered by his office painted “a disturbing picture of rampant state violence and targeted killings”.

“There are reasonable grounds to believe hundreds of extrajudicial killings, extensive arbitrary arrests and detentions, and torture, were carried out with the knowledge, co-ordination and direction of the political leadership and senior security officials as part of a strategy to suppress the protests.”

The report was requested by Bangladesh’s caretaker leader, Muhammad Yunus, who said he and his interim government remained “committed to transforming Bangladesh into a country in which all its people can live in security and dignity“.

The overall number of deaths given by the UN team is far higher than the 834 most recently estimated by his government.

The UN team that compiled the report included human rights investigators, a forensics physician and a weapons expert. Their findings are mainly based on more than 230 interviews with survivors, witnesses and others. They were given access to medical case files, photos, videos and other material.

“Former senior officials directly involved in handling the protests and other inside sources described how the former prime minister and other senior officials directed and oversaw a series of large-scale operations, in which security and intelligence forces shot and killed protesters or arbitrarily arrested and tortured them,” the report said.

It “found patterns of security forces deliberately and impermissibly killing or maiming protesters, including incidents where people were shot at point-blank range”.

Mohammad Ali Arafat, a former minister in Sheikh Hasina’s cabinet tasked wth negotiating with demonstrators, rejected the report’s findings, saying it was “preposterous” to suggest she had ordered protest leaders to be killed.

“The problem with relying on ‘testimonies’ from unnamed security officials at this time is their utter unreliability,” he told the BBC.

“These security officials, who themselves are in the dock for the alleged rights violations, would naturally point fingers at whoever the current government in Bangladesh wants to implicate.

“The UN would be mistaken to rely on such ‘cut-throat’ defences.”

While the report attributes most of the violence to government security forces, it also raises concerns about attacks on those perceived to be supporters of the former government, and against some religious and ethnic groups.

These must be investigated too, the UN Human Rights Office said.

Additional reporting by Ethirajan Anbarasan