Turnout has been high in Tanzania's most competitive general elections, officials say, as a new opposition coalition tries to end the governing party's 54-year grip on power.

In some areas, voting was extended to allow those still in queues to cast their ballots, officials added.

Opinion polls have put the governing Chama Cha Mapinduzi (CCM) party ahead, but the result is expected to be close.

Four opposition groups are backing one candidate, a former prime minister.

The BBC's Tulanana Bohela reports from the main city Dar es Salaam that usually busy streets and markets are deserted, and large queues have formed at polling stations as people wait patiently to cast their ballots.

President Jakaya Kikwete, who is standing down after two terms, has called for peace ahead of the election, adding that "anyone who tries to cause trouble will be dealt with".

CCM was formed in 1977 from a merger of two post-colonial parties and has effectively been in power since independence in 1961.

It has fielded Works Minister John Magufuli, 55, as its presidential candidate.

He is being challenged by Edward Lowassa, 62, who quit CCM after he failed to win its presidential nomination.

He is contesting the poll under the banner of the Ukawa coalition.

Both main presidential candidates have already cast their ballots.

"I've carried out my duty as a citizen by voting. My appeal to those Tanzanians who have not voted is to ensure they do so to elect the people they want," Mr Magufuli said.

Mr Lowassa said he was confident of victory, and urged people to vote peacefully.

Last week, he told the BBC Swahili service that he will "go back to his village to rear his cattle" if he loses.

The semi-autonomous island archipelago of Zanzibar is also voting for a president and local leaders.

It has been hit by violence in previous elections, unlike the mainland where elections tend to be peaceful.

The BBC's Sammy Awami in Zanzibar says that so far, there have been no reports of violence on the islands, with the voting process proceeding smoothly.