The first democratically-elected president of the Maldives has resigned and been replaced by his vice president after the police and army clashed in the streets amid protests over the arrest of a top judge.

Mohammed Waheed Hassan, who previously worked as a top Unicef official, was sworn in as the new Maldivian president in the afternoon. Soon after, the judge was released.

In a televised address, Hassan promised to protect his predecessor, Mohamed Nasheed, from retribution and called for the chaos in the streets to stop.

"I urge everyone to make this a peaceful country," he said.

Mr Nasheed's resignation marked a stunning fall for the former human rights campaigner who defeated the nation's long-time ruler in the country's first multiparty election. He was also an environmental celebrity, travelling the world to persuade governments to combat the climate change that could raise sea levels and inundate his archipelago nation.

Mr Nasheed resigned in a nationally televised address after police joined the protesters and then clashed with soldiers in the streets. Some of the soldiers then defected to the police side.

The latest protests in the Indian Ocean nation known for its lavish beach resorts erupted after Mr Nasheed ordered the military to arrest Abdulla Mohamed, the chief judge of the Criminal Court. The judge had ordered the release of a government critic he said had been illegally detained.

Mr Hassan - then the vice president - the Supreme Court, Human Rights Commission, Judicial Services Commission and the office of the United Nations High Commissioner for Human Rights all called for Mohamed to be released. Mr Nasheed's government accused the judge of political bias and corruption, said the country's judicial system had failed.

The crisis came to a head when hundreds of police demonstrated in the capital, Male, after officials ordered them to withdraw protection for government and opposition supporters protesting close to each other. The withdrawal resulted in a clash that injured at least three people.

Later, troops fired rubber bullets and clashed with the police. When Mr Nasheed visited the police and urged them to end the protest, they refused and instead chanted for his resignation. Mr Mohamed was released after Hassan took power.