The most violent scenes so far of the wave of protests sweeping the Arab world were seen in its most repressive country as Muammar Gaddafi appeared to be relying on brute force to crush what began last week as peaceful protests but may now threaten his 41-year rule.

Tensions eased in the Gulf state of Bahrain after troops withdrew from a square in central Manama occupied by Shia protesters. Thousands of security personnel were deployed in the Iranian capital, Tehran, to forestall an opposition rally. Elsewhere in the region unrest hit Yemen,Morocco, Oman, Kuwait and Algeria.

But the eyes of the world were on Benghazi and elsewhere in eastern Libya where shocked witnesses talked of "massacres" and described corpses shot in the head, chest or neck piling up in hospitals running short of blood and medicines.

Estimates of the total number of fatalities over six days of unprecedented unrest ranged from 173 to 285. Some opposition sources gave figures as high as 500.

Gaddafi's sons, Khamis and Saadi, and intelligence chief Abdullah Sanussi were reportedly commanding efforts to crush the protests in Benghazi, the country's second city, where buildings were ransacked and troops and police forced to retreat to a compound to pick off demonstrators with sniper and artillery fire.

Al-Sharq al-Awsat, the Saudi newspaper, quoted sources close to the Gaddafi family as saying they would "die on Libyan soil" rather than give up power like the presidents of Egypt and Tunisia.

Facts were hard to pin down in the face of a news blackout that included jamming of the signal of the al-Jazeera satellite TV network and interference with telephone and internet connections. But there were multiple claims of the army firing into crowds and the targeting of mourners at the funerals of those killed on Saturday. The Libya al-Yawm news website quoted one local doctor as saying that 285 people had died in Benghazi alone.

"Now people are dying we've got nothing else to live for," a student blogger told the Guardian. "What needs to happen now is for the killing to to stop. But that won't happen until he [Gaddafi] is out. We just want to be able to live like human beings. Nothing will happen until protests really kick off in Tripoli, the capital. It's like a pressure cooker. People are boiling up inside. I'm not even afraid any more. Once I wouldn't have spoken at all by phone. Now I don't care."

The US, Britain and the EU all expressed concern at the escalation in violence, but no punitive measures were announced. On Friday the UK revoked licences for the export of riot control equipment. Libya warned the EU on Sunday it would halt co-operation over illegal immigration unless the EU stopped supporting pro-democracy protests.

Libya, once treated as a pariah, has been embraced by western countries hungry for oil and lucrative investment opportunities since Gaddafi abandoned his support for terrorism but there has been very little easing of domestic repression.

William Hague, the foreign secretary, spoke to Gaddafi's reform-minded son Saif al-Islam and "expressed alarm at reports of large numbers of people being killed or attacked by Libyan security forces … The foreign secretary strongly encouraged the Libyan government to embark on dialogue and implement reforms."

Oliver Miles, a former British ambassador to Libya, writes on the Guardian's Comment is Free site: "Assuming that the Libyan protesters have the stamina and determination of those in Tunisia and Egypt , even in the face of gunfire, the resolution of the conflict seems to depend on two factors: will the disturbances spread to the different urban environment of Tripoli? And will the army – composed of Libyans, not foreign mercenaries, and therefore open to tribal influences which are largely unknown – continue to be willing to fire on unarmed civilians?"