Foreign embassies in Tanzania have raised concerns after the semi-autonomous Zanzibar archipelago annulled polls over irregularities, sparking tensions on the islands.

The United States embassy in Dar es Salaam said in a statement it was "gravely alarmed" at the annulment, while Britain's High Commissioner Dianna Melrose said she was "concerned" about Zanzibar's decision and made a "call for calm."

Zanzibar's electoral commission Wednesday said the vote on the Indian Ocean islands — where the 500,000 registered electorate also voted for Tanzania's national president — must be carried out again, citing "violations of electoral law".

READ: Zanzibar poll annulled, throws results into doubt

An AFP reporter on the islands' capital Stone Town said the streets appeared to be returning to normal and were quiet Thursday morning, although security forces maintained a heavy presence.

"This action halted an orderly and peaceful election, as evaluated by observer missions from the US Embassy, European Union, Commonwealth, and Southern Africa Development Community, and a tabulation process nearing completion," the US said in a statement.

"We call for this announcement to be recalled, and urge all parties to maintain a commitment to a transparent and peaceful democratic process. The people of Zanzibar deserve that."

READ: Observers to call for openness in Tanzania election

Opposition parties have also alleged rigging in Sunday's presidential, general and local elections, seen as the hardest fought polls in the east African nation, where the ruling party is facing its first major challenge in decades of dominance.

Despite Zanzibar's pullout, Tanzania's National Electoral Commission (NEC) has said it still expects to complete vote counting on the mainland Thursday and declare presidential results, with the ruling party presidential hopeful in the lead.

READ: Tanzania presidential election not affected by Zanzibar crisis

With 195 of 264 constituencies having released results in east Africa's most populous country, John Magufuli of the long-ruling Chama cha Mapinduzi had won 59.2 per cent of votes.

His nearest rival, Edward Lowassa of the opposition Chadema party, has 39.6 per cent, according to official results announced by the NEC late Wednesday.