Nairobi, KenyaTanzanian police have arrested Freeman Mbowe, the leader of the main opposition party Chadema, according to Tundu Lissu, the party's candidate in last week's presidential election.
Tanzania police arrest main opposition party leader ahead of protests
The opposition has demanded a repeat election, citing widespread irregularities, and called for protests against the outcome, which returned President John Magufuli to office with 84% of the vote.
Those arrested with Mbowe include former member of parliament Godbless Lema, the former mayor of Dar es Salaam, Isaya Mwita, and the former mayor of Ubungo municipality, Boniface Jacob.
"I got a message around midnight that they had been taken in," Lissu told Reuters on Monday.
Mwananchi, a privately-owned daily that publishes in Swahili, quoted Dar es Salaam's regional police commander Lazaro Mambosasa as saying they had detained Mbowe and other opposition leaders.
Magufuli was seeking a second five-year term in the poll.
The United States has said it was concerned about reports showing "systematic interference in the democratic process", while Britain said it was "troubled by the reports of violence and heavy-handed policing in the elections".
Elections took place simultaneously both in Tanzania and in the country's semi-autonomous state of Zanzibar, an Indian Ocean archipelago.
In Zanzibar, the CCM presidential candidate, Hussein Mwinyi, was declared the winner after securing 76% of the vote, the Zanzibar Electoral Commission announced. Seif Sharif Hamad, the opposition ACT-Wazalendo's presidential candidate there, and other leaders were arrested, his party said, after he called for protests earlier on Thursday.
"Detaining opposition leaders is not the act of a government confident in its electoral victory," US ambassador to Tanzania Donald Wright said in a tweet on Thursday. "Maalim Seif and his colleagues should be released immediately."
On the eve of the election and on election day, some Tanzanians reported disruptions when trying to access social media platforms -- accounts confirmed by Twitter and Internet blockage monitor NetBlocks, which reported widespread problems.
Lissu also accused the government of voting irregularities at polling stations.