Dozens of opposition supporters took to the streets of Tunisia's capital on Saturday to protest against the start of talks over a new constitution, which they see as an attempt by President Kais Saied to consolidate a grab for power.

Saied last year dismissed parliament and began ruling by decree in a move his opponents described as a coup, but he said was necessary to save Tunisia after a "decade of ruin".

The president, who has also replaced the judicial supreme council and the electoral commission, has pledged to hold a referendum on July 25 to vote on a new constitution, and on Saturday talks over drawing this up started in Carthage.

However, Saied has excluded all the main political parties, including the Islamist Ennahda and the Free Destourian Party - the largest two in the country. The powerful UGTT Union has refused to participate in the talks.

Dozens of supporters of the Ettakatol, Attayar, Workers and Republican parties took to the streets of Tunis on Saturday, saying the talks in Carthage were illegitimate.

The police prevented them, including some party leaders, from reaching the headquarters of the electoral commission.

"Tunisia has entered the path of falsifying the will of the voters and an atmosphere in which freedoms are not available," Republican Party leader Issam Chebbi told reporters.

The Ennahda and Free Destourian parties, a two rival parties said this week they would also step up protests, while the UGTT has called for a national strike on June 16 after the government refused to increase wages.

Washington this week accused Saied of undermining Tunisia's democratic institutions after he fired dozens of judges.

Three small parties and activists loyal to the president, some economists, the Union of Businessmen UTICA and the Human Rights League participated in the first session of the talks in Carthage.

A draft of the new constitution is due to be presented to Saied on June 20.