Tunisia's interior ministry said on Friday it had information about serious threats to the life of the president and that it had foiled what it called a separate attack near a synagogue, adding to concerns over a mounting political crisis.

The ministry said both internal and external elements were involved in plans targeting the president. Spokesperson Fadhila Khelifi said in a news conference that "the goal was to undermine Tunisian public security".

In what the ministry said was a separate incident, an attacker was arrested after injuring two police while targeting a security point outside a Tunis synagogue overnight, a security official said.

President Kais Saied's opponents accuse him of a coup for seizing most powers last summer to rule by decree and preparing a new constitution that he plans to put to a referendum next month.

Opposition to Saied's moves has broadened over recent months as nearly all major political parties as well as the powerful labour union have come out against his plans, holding street rallies against him.

However, while critics of the president say his moves have raised concerns over rights and freedoms won in the 2011 revolution that brought democracy, there has been no widespread crackdown on the opposition.

Saied says his moves are legal and were needed to save Tunisia from years of political paralysis and economic stagnation.

Tunisia has a small Jewish minority and hosts an annual pilgrimage to one of Africa's oldest synagogues, on the island of Djerba. An al Qaeda attack there in 2002 killed 21 visitors.