PARIS (Reuters) - A Lebanese-Canadian who is the main suspect in a 1980 bombing that killed four people outside a Paris synagogue was placed under formal investigation on Saturday, his lawyer told Reuters.

Hassan Diab arrived in Paris from Canada on Saturday after the country's highest court on Thursday refused to rule on his appeal against extradition to France. He will be detained until the start of his trial.

"My client has always said he is innocent, and will continue to do so," Diab's lawyer Stephane Bonifassi told Reuters.

Diab was arrested by Canadian police in 2008 but was released and lived under judicial supervision, his lawyer said. Before his arrest, Diab, 60, had become a Canadian citizen and was a lecturer on sociology at two Ottawa universities.

Diab is the suspect in the incident where a bomb was placed in a bag attached to a motorbike parked outside a synagogue in Rue Copernic, in the wealthy 16th arrondissement. It exploded on the final day of a Jewish festival, just before a crowd was due to emerge from the synagogue.

Diab has said the evidence was based on a flawed handwriting analysis comparing his writing with that found on a Paris hotel registration card in 1980.

French intelligence has said Diab had been a member of the Popular Front for the Liberation of Palestine, a resistance movement set up after the occupation of the West Bank by Israel in 1967.