NAIROBI (Reuters) - The United States said on Thursday there were “serious doubts” about the credibility of Tanzania’s presidential election while the leading opposition candidate urged countries not to recognise the results of a “travesty” of a poll.The U.S. Embassy in the East African country said there had been “credible allegations of significant election-related fraud and intimidation” in Wednesday’s poll in which voters were electing a president and lawmakers. The vote was marred by allegations of arrests of candidates and protesters, restrictions on agents of political parties to access polling stations, multiple voting, pre-ticking of ballots and widespread blocking of social media, the U.S. Embassy said. Elections took place simultaneously both in Tanzania and in the country’s semi-autonomous state of Zanzibar, an Indian Ocean archipelago. Tundu Lissu, the main challenger to incumbent President John Magufuli, told Reuters the election results - expected within a week - could not be trusted. Magufuli is seeking a second, five-year term in the poll. Preliminary results announced by the electoral commission showed Magufuli leading against his challengers. “The results should not be recognised by any country in the world, should not be recognised by the African Union and the Commonwealth,” Lissu told Reuters. He urged the world to take action against “those who perpetrated this travesty”. In Zanzibar, the CCM presidential candidate, Hussein Mwinyi, was declared the winner after securing 76% of the vote, the Zanzibar Electoral Commission announced. A version of the CCM has held power in Tanzania since independence from Britain in 1961.