Vladimir Putin is due to be sworn in for a fourth term as president of Russia on Monday after winning the last election in March. He has been in power for 18 years, whether as president or prime minister, and opponents have likened his tenure to the reign of a tsar, or emperor. Riot police confronted protesters against his rule in Moscow and other Russian cities on Saturday. There have been fears of new unrest on Monday as he takes office. The inauguration is likely to be low-key. Putin is only expected to meet volunteers who took part in his election campaign.