Republic of Congo opposition leaders called for a boycott of the Sunday referendum vote that could see the country's longtime president eligible for a new term.

Clement Mierassa and Emmanuel Bongouandza, leaders of two opposition parties, also said the opposition is ready for dialogue about the constitutional referendum.

Politicians campaigning against the Oct. 25 referendum had called for a campaign of "civil disobedience." Four people were killed on Tuesday when demonstrators attacked police stations.

Under current laws 71-year-old President Denis Sassou N'Guesso is barred from seeking re-election when his second term expires next year. The referendum calls for scrapping a two-term limit and 70-year-old age limit for candidates.