Tanzania's main opposition leader Freeman Mbowe was attacked by unidentified assailants as he entered his home late at night, his party said on Tuesday, the latest misfortune to befall the opposition ahead of a general election in October.

President John Magufuli, nicknamed "the Bulldozer," swept to power in 2015 promising an end to corruption. But he has been accused of curbing human rights including limiting freedom of expression and cracking down on leading opposition figures.

Mbowe, who is the leader of the opposition in parliament and chairman of the CHADEMA party, was set upon as he was returning home just after midnight, party officials said.

"Unknown people surrounded him and assaulted him before he started taking the stairs. Though these people carried firearms, they didn't use them," said CHADEMA's Secretary-General, John Mnyika.

Mbowe, who along with other opposition lawmakers was found guilty of sedition in March, was rushed to a hospital in the administrative capital Dodoma, where he was receiving treatment, Mnyika said, without giving more details.

Dodoma's police commander Gilesi Muroto was not immediately available for comment. There was also no immediate comment from the government.

Tanzania's ruling CCM party, and its predecessor TANU, have governed since independence from Britain in 1961.

Mbowe, who unsuccessfully ran for the presidency in the 2005 election, has not declared whether he will be a candidate in the October polls.

His deputy in CHADEMA, Tundu Lissu, has offered himself as a candidate in the presidential election. Lissu has been living in exile in Europe after he narrowly escaped assassination by unknown gunmen in Dodoma, back in September 2017.