ATHENS (Reuters) - Greece's anti-austerity leftist party Syriza is on course to win Sunday's snap election by a large margin over Prime Minister Antonis Samaras' conservatives, a joint exit poll showed.

Syriza received between 35.5 and 39.5 percent of the national vote, ahead of Samaras' center New Democracy party which took 23 to 27 percent, according to the poll by Metron Analysis, GPO, Alco, MRB, Marc.

Led by the 40-year-old Alexis Tsipras, Syriza wants to renegotiate a chunk of Greek debt and end austerity measures. That has spooked markets which fear a new financial crisis in Greece could push it out of the euro, though any broader repercussions for the bloc are expected to be limited.

The biggest party generally needs between 36 and 40 percent of the vote to win outright although the exact figure depends on the share of the vote taken by parties that fail to cross the 3 percent threshold to enter parliament. The exit poll indicated that seven or eight parties will make it into parliament.

Centrists To Potami and far-right Golden Dawn were tied for third spot with 6.4 to 8 percent of the vote, according to the exit poll.

(Writing by Deepa Babington)