The long-ruling CCM party ran almost entirely unopposed in local government polls held on November 24.
The opposition boycotted the vote, citing violence and intimidation that rights groups say have become a hallmark of Magufuli’s leadership.
His party clinched most of the 16,000 seats for street and village leaders – influential posts essential for grassroots campaigning ahead of next year’s presidential election, in which Magufuli is expected to run again.
Chadema, the main opposition party, refused to take part, saying its candidates were fearful or disqualified from running by stringent rules used to block them.
The party says its members have been kidnapped and beaten, and at least one has blamed authorities for an attack in 2017 in which he was shot multiple times. Five other smaller parties also joined the boycott.
Washington and London said Tanzania’s refusal to provide accreditation to respected election monitors ahead of the polls eroded confidence in the outcome.
“This troubling development calls into question the credibility of the election process and results,” the US embassy in Tanzania said in a statement.
The British high commissioner to Tanzania, Sarah Cooke, said in a statement that the “coordinated” disqualification of opposition candidates contributed towards the voters being denied their right to choose their own leaders freely and fairly.
Magufuli, a strongman elected in 2015 promising to crack down on corruption but whose tenure has been marred by attacks on his critics and the press.
Free media has been intimidated by draconian cybercrime laws, critical newspapers and bloggers have been silenced, and opposition members have been harassed, according to Human Rights Watch (HRW) and Amnesty International.
Erick Kabendera, a prominent journalist known for his unflinching coverage of Magufuli, was arrested in July and charged in a case rights group say is politically motivated.
A court in November postponed his trial for organised crime and financial offences for an eighth time.
Once a beacon of stability in the region, Tanzania fell 40 places this year on Reporters Without Borders’ Press Freedom Index.