* President's critics: New charter gives him too much power

* Analysts expect voters to back constitution

* Proposed changes draw little Western criticism

* Most Tunisians more preoccupied with economic problems

* Opposition parties are boycotting referendum (Updates turnout figures, adds analyst)

By Tarek Amara and Angus McDowall

TUNIS, July 25 (Reuters) - Tunisians voted on Monday on a new constitution which critics of President Kais Saied fear will dismantle the democratic gains of the 2011 revolution by handing him nearly total power.

The divided opposition urged a boycott, calling Saied's moves a coup that risks flinging Tunisia back into the autocratic era from before the revolution and putting the final nail into the coffin of the 2011 Arab Spring uprisings.

The electoral commission said 13.6% of voters had taken part by 1430 GMT, nine and a half hours into the 16-hour voting period. However, with very hot weather meaning a possible surge in evening voting, it was not clear what the figure meant for the eventual turnout.

The new constitution shifts power back to the presidency and away from parliament, which for years has been synonymous with political paralysis, and where an Islamist party, Ennahda, has been the biggest faction since the revolution.

Casting his vote, Saied hailed the referendum as the foundation of a new republic, a year to the day after he ousted an elected parliament, established emergency rule and began governing by decree.

Western democracies that looked to Tunisia as the only success story of the Arab Spring have yet to comment on the proposed new constitution, although they have urged Tunis over the past year to return to the democratic path.

"I'm frustrated by all of them. I'd rather enjoy this hot day than go and vote," said Samia, a woman sitting with her husband and teenage son on the beach at La Marsa near Tunis.

Others voiced support for Saied.

Casting his vote on Rue Marseilles in downtown Tunis, Illyes Moujahed said the former law professor was the only hope.

"I'm here to save Tunisia from collapse. To save it from years of corruption and failure," said Moujahed, first in line.

But the atmosphere was muted in the run-up to the referendum, with only small crowds attending rallies for and against the constitution.

Standing outside a cafe in the capital, Samir Slimane said he was not interested in voting. "I have no hope of change. Kais Saied will not change anything. He only seeks to have all the powers," he said.

INFIGHTING

Many Tunisians welcomed Saied's sudden seizure of power last summer as a corrective to years of political infighting and government failure by ruling factions. But he has done little to address deepening economic hardship and Tunisia is seeking an IMF rescue package.