Arusha - Tanzania's main opposition parties have signed a pack to mount a united challenge to the ruling party in presidential and general elections next year, they said in a statement.
Four key parties, which met on Sunday, said for the first time they would put up single candidates "at all levels", including for the presidential poll, due in October 2015.
President Jakaya Kikwete's ruling Chama Cha Mapinduzi (CCM) party has been in power since independence in 1961, and holds two-thirds of seats in the assembly.
Having served two terms in power, he cannot run again.
The opposition coalition is headed by Chadema party, the Civic United Front (CUF), the National League for Democracy (NLD) and the National Convention for Construction and Reform (NCCR).
Chadema, the main opposition party, also known as the Party for Democracy and Progress, has supporters mainly in the Tanzania's cities and towns.
The CUF is part of the ruling coalition on the autonomous archipelago of Zanzibar.
The four parties also agreed to jointly campaign against a proposed new constitution, which Tanzanians are due to vote on in a referendum in April.
The opposition is angry because the draft constitution fails to push their demands for greater federalism which would limit presidential powers.
While opposition parties have yet to name their choice for the presidential poll, there are several potential candidates for the ruling CCM party, including Prime Minister Mizengo Pinda.