* Egypt has been convulsed by protests, street violence
* Crisis was sparked by president expanding his powers
CAIRO, Dec 7 (Reuters) - Egypt's main opposition coalition said on Friday it would not take part in a dialogue proposed by President Mohamed Mursi to end a crisis sparked by the Islamist leader's decision to expand his powers.
Speaking in a televised address on Thursday night, Mursi had invited political leaders and others for talks on Saturday and said plans for a Dec. 15 referendum on a new constitution were on track despite clashes that killed seven people.
"The National Salvation Front is not taking part in the dialogue, that is the official stance," Ahmed Said, one of the leading members of the coalition who also heads the liberal Free Egyptians Party, told Reuters.
Prominent reformist Mohamed ElBaradei and the Wafd party, both members of the liberal-minded coalition, said they would not be taking part.
"I appeal to all national forces not to participate in dialogue that lacks the basics of real dialogue, we are for dialogue that is not based on arm-twisting and imposing a fait accompli," ElBaradei said in a message on Twitter.
Mursi's administration insists the president's actions were aimed at speeding up the transition to democracy that had been hampered by a judiciary still packed with appointees of ousted President Hosni Mubarak.
The Front has demanded that Mursi scrap his decree - that put his decisions beyond legal challenge - and postpone the referendum and redraft the constitution, which was fast-tracked through an Islamist-dominated assembly in the teeth of liberal opposition.
Said said the Front had come out against talks because Mursi had said he would press ahead with a referendum regardless and had prejudged the agenda by outlining parts of his decree that could be amended before talks had even begun.
"He has already set the ceiling of the discussion and has already made limits to it," Said said, saying the president was ignoring protests and violence on the street.
"Is this an environment for people to say 'yes' or 'no' to a document that is going to divide them rather than unite them?" he said, adding that voting abroad in the referendum was set to start on Saturday meaning it had already been set in motion.
The Foreign Ministry announced the Saturday start to voting abroad on its official Facebook page.
Another member of the Front, Amr Moussa, had also said the group was sticking to its demand that the referendum be delayed, amongst other demands.