Maldivian police raided the opposition campaign headquarters on Saturday, the opposition said, on the eve of Sunday's presidential election that international monitors fear will be rigged. Police stormed the Maldivian Democratic Party (MDP) office in Male during the afternoon without a warrant and carried out a search, the party said in a statement. For their part, police said in a brief statement that they acted to prevent "illegal activities." An MDP official outside the building said party workers were prevented from going in to the campaign headquarters, but no arrests had been made. Campaigning came to an end at 6:00 pm (1300 GMT) and any canvassing after the deadline is a criminal offence. The police action followed a street rally by MDP supporters carrying yellow flags of the party. President Abdulla Yameen's supporters also took to the streets in another part of the small capital city. Earlier in the day, the Asian Network for Free Elections (ANFREL) said the political environment in the tourist paradise was heavily tipped in favour of Yameen and they did not expect a fair contest. "The ANFREL denounces recent developments... which ensure that the upcoming presidential election of September 23 cannot be considered free and fair," the group said in a statement. It said it was recognised by the Maldivian Elections Commission and given accreditation to monitor the voting, but that its staff were denied visas to enter the country. A number of international journalists have also been denied permission to cover the polls. "It appears that Maldivian authorities are granting visas only to observers and monitors they perceive as friendly, while using ANFREL’s name and that of other applicants in an attempt to gain international legitimacy," the organisation said. The statement came a day after exiled former leader Mohamed Nasheed urged the international community not to accept the outcome of what he said would be a flawed ballot. Nasheed, the country's first democratically-elected leader, told reporters in neighbouring Sri Lanka that Yameen would "lose the election" but would "hold onto power" after rigging the electoral process. - 'Sombre events' - ANFREL also asked foreign governments to be vigilant about the election and predicted "sombre events" for the 340,000 Maldivians in a country otherwise known as a paradise for well-heeled tourists. Nasheed was forced to withdraw from the contest after the Maldives election commission disqualified him because of a 2015 terrorism conviction. The United Nations has said Nasheed's conviction and 13 year jail term were politically motivated and asked Yameen’s government to overturn the decision and pay him reparations. Nasheed has lived abroad since travelling out of the country on prison leave. His MDP is the only challenger in Sunday's vote after securing the support of all other opposition parties. A relatively unknown politician, 54-year-old Ibrahim Mohamed Solih, is backed by Nasheed to try and beat Yameen, who came to power following a controversial run off against Nasheed in the 2013 election. The second round of voting then was delayed, giving Yameen more time to pull together a coalition. The United States and European Union have expressed deep concern over Yameen's actions, and rights activists have called for sanctions on the president and his aides.