At least 208 people are believed to have been killed in Iran during a security crackdown on protests that came after a surprise petrol price increase, Amnesty International has said.

In a statement on Monday, the United Kingdom-based rights group said the "alarming" death toll was based "on credible reports" it had received, adding that the actual figure was likely to be higher.

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Amnesty said there have been "dozens of deaths" in Shahriar city in Tehran province. It did not provide a location breakdown for the number of people killed in other parts of the country.

Iran, which has not given a definitive death toll, has previously rejected Amnesty's figures.

Demonstrations broke out in mid-November following an abrupt decision by authorities to roll out a petrol-rationing scheme and slash subsidies as part of efforts to offset the effect of punishing US sanctions. The move sent prices by at least 50 percent.

Iran has accused "thugs" linked to its rivals for the unrest, which it described as the work a "very dangerous conspiracy". Some 200,000 people took part in the protests, according to officials, while hundreds of banks and government sites were torched. A legislator was quoted as saying this week that 7,000 people were arrested by authorities, who also shut down the internet for almost a week.

'Horrific killing spree'

In its statement, Amnesty said it had verified and analysed extensive video footage that showed security forces shooting at unarmed protesters.

"The massive use of lethal force was a way of nipping the protests in the bud," Raha Bahreini, Amnesty's Iran researcher, told Al Jazeera.

"The escalation of the use of force is evident from the fact that protesters were shot from close range. Protesters were also shot from the rooftops and in one case from a helicopter. This represents a regression even by the Iranian government's own standards."

Amnesty said it had ascertained the credibility of the reports by interviewing a range of sources inside and outside the countries including relatives of the victims, journalists and human rights activists involved in collecting the information.

According to its information, families of victims were threatened and warned not to speak to the media, or to hold funeral services, Amnesty said.

It described the death toll as "evidence that Iran's security forces went on a horrific killing spree" and urged the international community to help ensure accountability.

"Unless the authorities are held to account by the United Nations and the international community, the unlawful killing will worsen and we will see more bloodshed," Bahreini said.

Meanwhile, Omid Memarian, deputy director at the US-based Centre for Human Rights in Iran, called on Tehran to allow Javaid Rehman, the UN special rapporteur on the situation of human rights in Iran, to enter the country and "investigate these crimes".

"Iran has repeatedly failed to investigate cases of rights violations in the past decades and to hold perpetrators accountable," he told Al Jazeera.

Iran statement

For its part, the Permanent Mission of the Islamic Republic of Iran to the UN in Geneva said in a statement to Al Jazeera that Tehran "has good reasons to suspect the credibility of the reports released by AI [Amnesty International] due to its past pattern of over-reliance on discredited and unreliable sources and because of certain in-built biases concerning Iran."

It added: "Iran fully respects the right to peaceful assemblies.

"The fact that hundreds of law enforcement and police forces plus innocent citizens were among the casualties and that the security authorities exercised maximum restraint and care even in dealing with those who abused the protests to undermine public safety and damage public and private property is testimony to this."