* Presidency says talks failed, blames Muslim Brotherhood
* U.S. envoy leaves Cairo without comment
* U.S. senators forecast bloodshed within weeks if no dialogue
* Stage set to clear pro-Mursi sit-ins by force (Recasts, adds Dutch minister, International Crisis Group)
By Angus MacSwan and Tom Perry
CAIRO, Aug 7 (Reuters) - Egypt's political crisis entered a tense new phase on Wednesday after international mediation efforts collapsed and the army-installed government repeated its threat to take action against supporters of deposed President Mohamed Mursi.
Both sides called their supporters on to the streets on Thursday and in two protest camps in Cairo, Mursi supporters strengthened sandbag-and brick barricades in readiness for any action by security forces.
The U.S. envoy made his way home after days of trying to broker a compromise between the government and Mursi's Muslim Brotherhood. The European Union special envoy stayed on but Brussels said it was very concerned by the breakdown in talks.
"What I see is that confrontation is mounting and that more people will turn to the streets to protest and the tendency in the armed forces to repress that will mount," said Dutch Foreign Minister Frans Timmermans, one of a host of foreign officials who have visited Cairo as the crisis unfolded.
"So I think there's need to be worried about the next days and weeks," he said.
The army ousted the Islamist Mursi, Egypt's first freely elected leader, on July 3 after huge street demonstrations against his rule.
Mursi and leaders of his Muslim Brotherhood have been rounded up and detained. But thousands of their supporters have demonstrated to demand his reinstatement.
Almost 300 people have been killed in political violence since the overthrow, including 80 Mursi supporters shot dead by security forces in a single incident on July 27.
President Adli Mansour's office said the push to defuse the crisis by envoys from the United States, European Union, Qatar and the United Arab Emirates had ended in failure on Tuesday.
The presidency blamed the Muslim Brotherhood for the breakdown, and for any events that might result from this "related to breaches of the law and endangering civil peace".
Soon after, interim Prime Minister Hazem el-Beblawi said the government's decision to dismantle the protest camps was final and its patience had nearly expired.
Beblawi accused protesters of inciting violence, blocking roads and detaining citizens, and he warned that any further violence would be met "with utmost force and decisiveness."
People should leave the camps now, he said, adding that those whose hands were not "sullied with blood" would not face legal action.
Muslim Brotherhood spokesman Gehad El-Haddad, asked about the threat, told Reuters: "This means they are preparing for an even bigger massacre. They should be sending us positive signals, not live bullets."
PEACE AT EID?
On Wednesday afternoon, people streamed into the camp outside Rabaa al-Adawiya mosque in northeast Cairo, where demonstrators have built barricades and armed themselves with sticks and rocks. Their numbers included many women and children.
"We will not leave until we get Mursi back," said Salma Imam, 19, student at Al-Azhar university. "It's not a government, the real government was chosen by the Egyptian people one year ago. This is not a legal government."
Any action could still be some time away, however.
Egyptians celebrate the Eid al-Fitr holiday marking the end of the Islamic fasting month of Ramadan from Thursday to Sunday, an inauspicious time for any act of violence.
And Egypt's leading Islamic authority on Wednesday announced plans to host talks on the crisis after Eid, which might also forestall an assault by the security forces.
"There are some initiatives that can be built upon to start national reconciliation," an al-Azhar official told the state news agency MENA.
Mursi's downfall was driven by fears he was trying to establish an Islamist autocracy, coupled with a failure to ease economic hardships afflicting most of Egypt's 84 million people.
The army says it was acting at the behest of the people and has lain out its own transition plan for new elections, a move rejected by the Brotherhood.
Hamdeen Sabahi, a leftist who came third in last year's presidential election, said the Islamists were in a state of denial about what had happened.
"The Muslim Brotherhood must accept the will of the people. I can't imagine any political solution," he said in a radio interview.
Pro-Mursi parties and leftists who backed his removal called rival demonstrations for Thursday, making the public holiday a potential flashpoint.
The National Alliance to Support Legitimacy, which includes the Brotherhood, urged Mursi supporters to take to the streets for an "Eid of Victory".
The leftist Popular Current party called for public Eid prayers in Tahrir Square, centre of the 2011 uprising that ousted long-ruling strongman Hosni Mubarak and set in train the current turmoil.
The International Crisis Group, a non-government organisation, said the most likely path in Egypt looked like confrontation amid political paralysis.
"It will take a Herculean effort to break out of this cycle; most of all, it will require all parties to go against type and act against their natural instincts," it said in an analysis.
Egypt is the Arab world's largest country, a bulwark in the United States' Middle East policy, and maintains an uneasy peace with Israel.
Dutch Foreign Minister Timmermans told Reuters that whatever happened in Egypt would have ramifications for other Arab countries struggling for democracy.
"And of course it's just across the Mediterranean from Europe so continued instability in Egypt will have an impact on the wider Mediterranean region and thus on Europe," he said.
(Additional reporting by Tom Perry, Tom Finn, Maggie Fick, Shadia Nasralla and Michael Georgy in Cairo and Paul Taylor in Paris; Editing by Sonya Hepinstall)