A leading opposition figure in Belarus has gone missing, shortly after witnesses said she was bundled by masked men into a minibus in Minsk. Maria Kolesnikova's whereabouts are unknown. Police have denied detaining her, the Interfax news agency reports. Ms Kolesnikova was one of three women who joined forces to challenge incumbent Alexander Lukashenko in August's presidential election. Mass protests followed his re-election amid allegations of vote-rigging. More than 600 people were arrested on Sunday, following a fourth consecutive weekend of anti-government protests in the capital Minsk and other cities. At least four people have died and hundreds have been injured as authorities have tried to crush dissent. On Monday the EU urged Belarus's government to free those jailed, calling the arrests "arbitrary and unexplained". EU leaders do not recognise the results of the election and have agreed to impose sanctions on Belarus. EU diplomats told Reuters news agency that the measures targeted 31 senior Belarus officials, including the interior minister. Mr Lukashenko - who has been in power since 1994 - has accused Western nations of interference. On Monday the Kremlin announced he would visit Moscow for talks "in the coming days". Russia has long been a close ally of Mr Lukashenko.