Tanzania’s president, John Magufuli, has won re-election, the national electoral commission said on Friday, after a contest that has been .

Magufuli received 12.5m votes in Wednesday’s presidential ballot, while his main challenger, Tundu Lissu of the Chadema party, received 1.9m votes, the commission said.

Magufuli has been seeking a second five-year term and promised voters that he would boost the economy by completing ambitious infrastructure projects started in his first term.

Lissu previously said that he will not accept the election results.

The US assistant secretary of state for African Affairs, Tibor Nagy, said on Friday: “We remain deeply concerned about reports of systematic interference in the democratic process.”

Dozens of opposition party officials and members were arrested in Zanzibar on Thursday and at least one was in hospital with severe injuries after allegations he was beaten by the police. The police have not commented on the incident.

The US embassy in the east African country said on Thursday there had been “credible allegations of significant election-related fraud and intimidation” in Wednesday’s vote for a president and lawmakers.

The election was marred by allegations of arrests among candidates and protesters, restrictions on the ability of political parties’ agents to access polling stations, multiple voting, pre-ticking of ballots and widespread blocking of social media, the US embassy said.

Officials at the electoral commission were not immediately available for comment. On Wednesday, the commission denied allegations of fake ballots, saying they were unofficial and unsubstantiated.

Zitto Kabwe, the leader of the main opposition party in Zanzibar, ACT-Wazalendo, and Chadema’s leader in parliament, Freeman Mbowe, are among dozens of opposition candidates who have lost their seats to the ruling party.