* Protesters break through barbed wire barricades
* Deputy says president could delay constitution vote
* Opposition demand end to Mursi's expanded powers
* Brotherhood says opposition out to topple elected leader
By Edmund Blair and Yasmine Saleh
CAIRO, Dec 7 (Reuters) - Tens of thousands of Egyptian protesters surged around the presidential palace on Friday and the opposition rejected President Mohamed Mursi's call for dialogue to end a crisis that has polarised the nation and sparked deadly clashes.
The Islamist leader's deputy said he could delay a Dec. 15 referendum on a constitution that liberals opposed, although the concession only partly meets a list of opposition demands that include scrapping a decree that expanded Mursi's powers.
"The people want the downfall of the regime" and "Leave, leave," crowds chanted after bursting through barbed wire barricades and climbing on tanks guarding the palace of Egypt's first freely elected president.
Their slogans echoed those used in a popular revolt that toppled Mursi's predecessor Hosni Mubarak in February 2011.
Vice President Mahmoud Mekky said in a statement sent to local media that the president was prepared to postpone the referendum if that could be done without legal challenge.
The dialogue meeting was expected to go ahead on Saturday in the absence of most opposition factions. "Tomorrow everything will be on the table," a presidential source said of the talks.
The opposition has demanded that Mursi rescind a Nov. 22 decree giving himself wide powers and delay the vote set for Dec. 15 on a constitution drafted by an Islamist-led assembly which they say fails to meet the aspirations of all Egyptians.
The state news agency reported that the election committee had postponed the start of voting for Egyptians abroad until Wednesday, instead of Saturday as planned. It did not say whether this would affect the timing of voting in Egypt.
Ahmed Said, leader of the liberal Free Egyptians Party, told Reuters that delaying expatriate voting was made to seem like a concession but would not change the opposition's stance.
He said the core opposition demand was to freeze Mursi's decree and "to reconsider the formation and structure of the constituent assembly", not simply to postpone the referendum.
The opposition organised marches converging on the palace which elite Republican Guard units had ringed with tanks and barbed wire on Thursday after violence between supporters and opponents of Mursi killed seven people and wounded 350.
Islamists, who had obeyed a military order for demonstrators to leave the palace environs, held funerals on Friday at Cairo's al-Azhar mosque for six Mursi partisans who were among the dead. "With our blood and souls, we sacrifice to Islam," they chanted.
In a speech late on Thursday, Mursi had refused to retract his Nov. 22 decree or cancel the referendum on the constitution, but offered talks on the way forward after the referendum.
The National Salvation Front, the main opposition coalition, said it would not join the dialogue. The Front's coordinator, Mohamed ElBaradei, a Nobel peace laureate, dismissed the offer as "arm-twisting and imposition of a fait accompli".
Murad Ali, spokesman of the Brotherhood's Freedom and Justice Party (FJP), said opposition reactions were sad: "What exit to this crisis do they have other than dialogue?" he asked.
Mursi's decree giving himself extra powers sparked the worst political crisis since he took office in June and set off renewed unrest that is dimming Egypt's hopes of stability and economic recovery after nearly two years of turmoil following the overthrow of Mubarak, a military-backed strongman.
The turmoil has exposed contrasting visions for Egypt, one held by Islamists, who were suppressed for decades by the army, and another by their rivals, who fear religious conservatives want to squeeze out other voices and restrict social freedoms.
Caught in the middle are many of Egypt's 83 million people who are desperate for an end to political turbulence threatening their precarious livelihoods in an economy under severe strain.
"We are so tired, by God," said Mohamed Ali, a labourer. "I did not vote for Mursi nor anyone else. I only care about bringing food to my family, but I haven't had work for a week."
A long political standoff will make it harder for Mursi's government to tackle the crushing budget deficit and stave off a balance of payments crisis. Austerity measures, especially cuts in costly fuel subsidies, seem inevitable to meet the terms of a $4.8-billion IMF loan that Egypt hopes to clinch this month.
U.S. President Barack Obama told Mursi on Thursday of his "deep concern" about casualties in this week's clashes and said "dialogue should occur without preconditions".
The upheaval in the most populous Arab nation worries the United States, which has given billions of dollars in military and other aid since Egypt made peace with Israel in 1979.
The conflict between Islamists and opponents who each believe the other is twisting the democratic rules to thwart them has poisoned the political atmosphere in Egypt.
The Muslim Brotherhood's spokesman, Mahmoud Ghozlan, told Reuters that if the opposition shunned the dialogue "it shows that their intention is to remove Mursi from the presidency and not to cancel the decree or the constitution as they claim".
Ayman Mohamed, 29, a protester at the palace, said Mursi should scrap the draft constitution and heed popular demands.
"He is the president of the republic. He can't just work for the Muslim Brotherhood," Mohamed said of the eight-decade-old Islamist movement that propelled Mursi from obscurity to power.