Chadema and fellow opposition party ACT-Wazalendo rejected results which saw President John Magufuli win a second term with 84 percent of results and his ruling party take 97 percent of seats in parliament.
Chadema chairman Freeman Mbowe had called for countrywide protests starting Monday as well as fresh elections.
“We have arrested Mbowe, he is in our hands,” said Dar es Salaam police chief Lazaro Mambosasa, adding six other top members of the party were also under arrest.
“On top of these protests which we banned, they wanted to incite the vandalisation of buildings, burn petrol stations, markets, and public transport vehicles,” he said.
“Any person implicated in the planning of illegal protests will be arrested and will face justice.”
Chadema’s presidential candidate Tundu Lissu won only 13 percent of the vote, after denouncing widespread fraud and intimidation of the opposition during the election and following years of repression and jailing of government opponents.
Lissu, who returned to Tanzania in July after three years abroad recovering from 16 bullet wounds sustained in an assassination attempt, said his party’s agents had been kicked out of polling stations and ballot boxes had been stuffed.
The result of presidential elections cannot be contested in Tanzania although the parliamentary outcome can be challenged.
“The door is closed for us to challenge the presidential results in court, and that is why we have decided to take this to the people, who have the power,” Lissu said.
Magufuli was first elected in 2015, and alarm has grown over his slide into autocracy amid a crackdown on the opposition and the media.
In semi-autonomous Zanzibar, which also elects its own president, the opposition’s presidential candidate was arrested twice last week, and attempts to protest were quickly and brutally crushed by security forces.
-AFP