By Renee Maltezou

ATHENS (Reuters) - Greece's lawmakers voted to revamp the electoral law and abolish a system of bonus seats on Thursday, in a move authorities say will better reflect public opinion in the crisis-hit nation.

At present, a bonus of 50 seats is offered to the party that wins the most votes in a national election, irrespective of how thin or wide the margin of victory is.

The law also lowers the voting age by a year to 17, a move the Interior Ministry said will 'sharpen democratic reflexes' among youngsters.

The reforms, which were a pre-election pledge of leftist Prime Minister Alexis Tsipras, may heighten political uncertainty in a country which has held four snap elections since 2012. They produced no outright winners but fragile coalitions in a fractious political landscape.

"If one considers that under present circumstances grand coalitions are pretty remote, it's obvious the new law creates more uncertainty because three or more parties would be required to create a viable government," said political analyst Costas Panagopoulos, head of the ALCO polling agency.

Tsipras and his coalition partner, the right-wing Independent Greeks, are clinging to a narrow majority with 153 seats in parliament, against 147 for the opposition.

Tsipras' Syriza party has been ranking second in opinion polls but is more popular than the conservative opposition among younger age groups.

Lowering the voting age was a trend in the European Union and would help restore confidence in a political system that many Greeks blame for their protracted debt crisis, authorities said.

"Our proposal not only addresses an historical demand to boost democracy. It also satisfies a contemporary demand of Greek people's majority for our political system to be reformed and rejuvenated," Tsipras said before the vote.

Polling at 4-5 percent for years, Syriza stormed to power for the first time in January 2015 and was re-elected in September after signing up to a third international bailout and agreeing to more austerity.

Under Greek statutes, a threshold of 200 votes would be required for the abolition of the 50-seat bonus to take effect from the next election. The measure won a simple majority of 179 which means the change would take effect two elections from now.

The government secured a majority of 151 which was needed for the lowering of the voting age, which can come into effect immediately.

The left-led coalition of 153 got the backing of the Union of Centrists party, which has 9 seats. The conservative New Democracy party rejected the bill.

Young Greeks have been hit especially hard by the debt crisis, with more than half of those aged 15-24 out of work, the highest proportion in Europe.

"We chose to give younger Greeks a voice, a more essential role, since this is the generation which risks suffering the impact of the crisis," the bill said.

(Reporting by Renee Maltezou, Additional reporting by Angeliki Koutantou,; Editing by Mark Trevelyan and Andrew Hay)