The air in Cairo was filled with smoke, as Susana Mendonca reports
Egypt in crisis
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Seven people have been killed in Cairo in overnight clashes between security forces and supporters of Egypt's ousted President Mohammed Morsi.
Police used tear gas to drive back protesters, some hurling rocks, who had blocked a main route in the capital.
The clashes came as a senior US envoy visited Egypt, saying it had been given a "second chance" at democracy.
William Burns met interim leaders but was snubbed by rival groups, including Mr Morsi's Muslim Brotherhood.
Mr Morsi was ousted on 3 July in what many have said was a military coup. The army says it was fulfilling the demands of the people after mass anti-Morsi protests.
Monday's battles erupted after hundreds of protesters, mostly members of Mr Morsi's Muslim Brotherhood Islamist movement, blocked the Sixth of October bridge, a major route over the Nile and through the capital, and the nearby Ramses Square, a transport hub.
AnalysisLyse DoucetChief international correspondent
Egypt's troubled transition to democracy has also been a rocky road for the US. During the historic uprising of 2011, Washington vacillated over which side to back.
Now, after the ousting of President Morsi, it is still criticised on both sides of a bitter Egyptian political divide. The anti-Morsi camp charges the US with backing the Muslim Brotherhood. The pro-Morsi camp accuses it of siding with the military.
Recently, the US developed a good working relationship with the ruling Muslim Brotherhood but found itself increasingly drawn into the deepening crisis between Islamists and more liberal forces.
The US now needs assurances that Egypt is still on the road to civilian rule, and its peace treaty with Israel is intact. If not, about $2bn, mostly annual military aid, will be at risk, as well as a vital relationship which, despite the strains, all sides still want to keep.
Read more from Lyse
Police fired tear gas to try to break up the blockade. Protesters responded by throwing stones. Other clashes broke out in the Giza district, in the south-west.
The head of Egypt's emergency services, Mohamed Sultan, said two people had died on or around the bridge and five in Giza.
State media quoted health ministry official Khaled al-Khatib as saying 261 people were injured in the clashes, which lasted into the early hours of Tuesday. Mr Khatib said 124 people were still in hospital.
Mr Sultan said security personnel were among the casualties.
Members of the Brotherhood said security forces had used live ammunition, and accused them of attacking a peaceful protest.
"We were praying. Suddenly there was shouting. We looked up and the police were on the bridge firing tear gas down on us," protester Adel Asman told Reuters news agency.
State media later quoted officials as saying 401 people had been arrested for "provoking unrest".
'Get out, Sisi'
The latest clashes are the most violent since last week, when more than 50 Morsi supporters were killed in fighting with troops outside the Republican Guard compound, where they believe the former president is being held.
The clashes in Cairo broke out as supporters of the ousted president blockaded the Sixth of October bridge, a major route through the capital.Continue reading the main story
The interim government has since announced its transition plan. A panel is to be formed by next week to draw up amendments to the constitution and a timetable for new elections.
Egyptian media reactions
Muhammad Salmawi in Egypt's liberal Al-Masry Al-Youm writes: "The US Administration should stop this silly talk about a 'coup'. If the US administration wants to repair its relationship with the Egyptian people, it should show respect for the Egyptians and recognise their right to choose their rulers on their own."
Abdallah Hilal in Egypt's pro-Muslim Brotherhood daily Freedom and Justice: "Oil countries would not dare to offer Egypt billions unless they had the approval of the US, ie Zionism."
Rafiq Khuri in Lebanon's Al-Anwar: "After a period of confusion, President Barack Obama's Administration has chosen to reposition itself... The superpower, which had put its money on the rule of the Muslim Brotherhood and the wave of political Islam as the region's destiny and a chance for US interests, is as usual playing both sides of the card."
Hamid al-Mansuri in United Arab Emirates' Al-Ittihad: "At the beginning of the Arab spring, the US showed willingness to put its money on the rule of Islamists in some Arab countries. Today, after the failed experience of the Muslim Brotherhood in Egypt, the US has become more accepting of the dominant Gulf viewpoint, which opposes the ideas of the Muslim Brotherhood and their rise to power."
But the Brotherhood has said it will not join a transitional government. Morsi supporters are demanding his reinstatement and have been holding a round-the-clock vigil outside the Rabaa al-Adawiya mosque, in the east of the capital, and at Cairo University in Giza.
Large crowds were again gathered at the mosque on Monday as Mr Burns, the US deputy secretary of state, visited.
"Get out, Sisi," some shouted, referring to the head of the armed forces, General Abdel Fattah al-Sisi, who oversaw the overthrow of Mr Morsi.
Mr Burns met interim President Adly Mansour and Prime Minister Hazem al-Beblawi, as well as Gen Sisi.
He described the events of the last two weeks as a "second chance to realise the promise of the revolution" that ended the authoritarian presidency of Hosni Mubarak in 2011.
He called on the military to avoid "politically motivated arrests", saying the US remained committed to an Egypt that was "stable, democratic, inclusive and tolerant".
But he insisted the US had "not come to lecture anyone. We will not try to impose our model on Egypt."
The envoy's comments come amid what correspondents say is an increasing antipathy towards the US among Egyptians on both sides of the political divide.
The US has stopped short of calling the army intervention a coup - doing so would trigger a legal stop of the some $1.5bn (£1bn) in the mostly military aid it sends to the country each year.
Adly Mansour's transition timeline
Panel formed to review constitution
Constitutional amendments to be finalised and put to referendum in four months
Parliamentary elections to be held by early 2014
Presidential elections to be called once new parliament convenes
Key players in the Egyptian crisis
Is Egypt heading for holy war?
But it has called for Mr Morsi to be released. He is being held at an undisclosed location and is being investigated on charges including inciting violence. A number of Brotherhood members have been arrested and warrants have been issued for many more.
Mr Burns said he planned to meet religious and civilian leaders, the heads of political parties and business figures during his two-day visit.
But both the ultra-conservative Salafi al-Nour party and the Tamarod anti-Morsi protest movement turned down invitations to meet Mr Burns, while the Muslim Brotherhood also said it had no plans to see him.
"Such kind of visit doesn't mean anything for us because we believe that America supported this military coup," Dina Zakaria, a member of the Brotherhood and its political wing the Freedom and Justice Party, told the BBC.
Islam Hammam, a Tamarod organiser, said the movement had turned down the invitation to talks with Mr Burns "because the United States did not stand with the Egyptian people from the beginning."