Syrian security forces have committed "crimes against humanity" since widespread anti-government protests began in March, according to a damning UN report (pdf) that will add to the mounting pressure on the president, Bashar al-Assad.

It said that least 256 children had been killed by government forces.

The investigation by the UN's independent international commission found patterns of summary execution, arbitrary arrest, enforced disappearance and torture, including sexual violence and abuse, some of it directed against children.

The UN report was published on a day that saw Syria lambast the Arab League for imposing unprecedented economic sanctions because of the refusal by Damascus to accept observers to protect civilians.

Further pressure came from France's foreign minister, Alain Juppé, who said Assad's days were "numbered". Britain condemned the "horrific and shocking actions carried out by the Assad regime against its own civilian population".

The 39-page document includes testimony from defectors from the security forces, who described indiscriminate shooting at unarmed protesters and snipers targeting those using loudspeakers or carrying cameras and mobile phones. A defector described the shooting of a two-year-old girl by an officer who said he did not want her to grow up to be a demonstrator. The figure of 256 children killed up to 9 November was attributed only to unspecified "reliable sources".

The report was based on interviews with 223 victims and witnesses, but investigators were denied access to the country, despite repeated requests.

According to UN figures issued this month, at least 3,500 people have been killed in Syria since March. The regime gives a figure of around 600. It says many of those were security personnel who were the victims of "armed terrorist gangs".

The report says: "State forces shot indiscriminately at unarmed protesters. Most were shot in the upper body, including in the head." Defectors told the commission that they had received orders to shoot at unarmed protesters without warning.

"In some instances, however, commanders ordered protesters to disperse and issued warnings prior to opening fire. In some cases, non-lethal means were used prior to or at the same time as live ammunition."

Snipers, it says, were responsible for many casualties, including people who were trying to rescue the wounded and collect the bodies of demonstrators. Defectors witnessed the killing of comrades who refused to fire at civilians.

Torture and killings reportedly took place in the Homs military hospital by security personnel dressed as doctors and allegedly acting with the complicity of medical staff. Torture was described as "rampant" at the detention facilities of the notorious air force intelligence branch at the Mazzeh airport near Damascus.

The commission said it was "particularly disturbed over the extensive reports of sexual violence, principally against men and boys, in places of detention". A 40-year-old man saw the rape of an 11-year-old boy by three security officers.

The report recommends "prompt, independent and impartial investigations under both domestic and international law to end impunity, ensure accountability and bring perpetrators to justice".

Diplomats said the Arab League could ask the UN security council to debate the abuses, though any referral to the international criminal court would require the support of Russia and China, which is unlikely to be forthcoming.

The three members of the UN commission were Brazilian, Turkish and American.

Syria's embattled government seems unlikely to respond. The official media attacked the Arab League measures as a "conspiracy" and warned they would hurt ordinary people. The foreign minister, Walid al-Moallem, said: "The Arabs don't want to admit the presence in Syria of groups of armed terrorists who are committing these crimes, abductions and attacks on public places."

Most Syrian assets had already been withdrawn from Arab countries in anticipation of the league's move, he said.

Analysts say that even without the full co-operation of Iraq and Lebanon, which did not back the measures, the sanctions are likely to have a severe impact on Syria's economy, especially in conjunction with EU ban on Syrian oil exports which came into force earlier in November. Turkey – not a member of the Arab League – is expected to announce its own sanctions.

"Despite diplomatic assertions that the sanctions are designed to avert recourse to overt foreign intervention in Syria, if they do not have an impact on the regime's behaviour, talk of more direct – perhaps military – measures is likely to increase," said David Hartwell, of the IHS consultancy in London.