Tanzania's electoral commission said it had disqualified opposition presidential candidate Luhaga Mpina for the second time on Monday, reversing a decision made two days earlier that had approved his nomination.

The disqualification of Mpina, who leads the country's second-largest opposition party, leaves President Samia Suluhu Hassan a clear run at next month's election, with only candidates from minor parties allowed to compete.

The vote will also be held without leading opposition party CHADEMA, which was disqualified in April after failing to sign the electoral code of conduct as part of its call for electoral reforms.

The CHADEMA presidential candidate, Tundu Lissu, has been in jail for over five months after being charged with treason in April. He has denied the charges.

Rights groups like Amnesty International say Lissu's detention and the unexplained abductions of government critics in recent months point to a government crackdown ahead of the election.

Over the weekend the Independent National Electoral Commission (INEC) accepted the nomination papers of Mpina, a former ruling party lawmaker, following a high court ruling that overturned an earlier decision by INEC to disqualify him.

"It is our hope the commission will drop all objections against our candidate so that he can proceed with the election campaigns in a bid to lead Tanzanians," said Shangwe Ayo, ACT-Wazalendo's deputy spokesperson, in a statement.