* Sustain face, chest and back injuries
* Island has seen street protests over past year
* Zanzibar is a popular tourist destination
By Fumbuka Ng'wanakilala
DAR ES SALAAM, Aug 8 (Reuters) - Men riding a motorbike threw acid at two British teenage girls in Tanzania's semi-autonomous Zanzibar region, leaving them with facial, chest and back injuries, a senior police official said on Thursday.
The pair, both 18 and from England's northern city of Manchester according to police in the Indian Ocean archipelago, were flown to Tanzania's commercial capital Dar es Salaam.
They had been volunteering at a local school in Zanzibar, an island that is popular with international tourists but has suffered a wave of deadly protests last year as supporters of an Islamist group repeatedly clashed with the police.
Britain is concerned about Wednesday's attack and is "in contact with the Tanzanian authorities", the Foreign Office said in a statement.
The police described the attack as "an isolated incident", refusing to link it to rising religious tension on the island between majority Muslims and its Christian population.
"The attackers approached the girls as they were walking on a street at around 7:15 p.m. and threw acid at them," Zanzibar Urban West regional police commander Mkadam Khamis Mkadam told Reuters. "The incident occurred when the streets were deserted as most people were breaking their Ramadan fast."
Television images showed one girl obviously in pain in the back of a car at the Zanzibar airport.
"The victims sustained facial, chest and back injuries from the acid attack," Mkadam said.
The Britons were expected to fly home on Thursday.
The attack comes during the tourist season in the historic town and after a Zanzibar Muslim leader, Sheikh Fadhil Suleiman Soraga, was hospitalised with acid burns in a November attack.
Two Christian leaders were killed early this year in separate attacks.
A separatist group in Zanzibar, Uamsho (Awakening), is pushing for the archipelago to exit from its 1964 union with mainland Tanzania, which is ruled as a secular country. Uamsho wants to introduce Islamic Sharia law in Zanzibar.
Supporters of the group have engaged in running street battles with the police in the past, but authorities have not linked the group with the attacks on Christian clerics. (Editing by Drazen Jorgic and Elizabeth Piper)