Image copyright Getty Images Image caption Mr Yu is a prominent human rights lawyer

A prominent Chinese human rights lawyer has been detained while walking his child to school, his family says.

Yu Wensheng, a vocal critic of China's ruling Communist Party, was seized by about a dozen people, including a Swat team, after he left his flat with his son on Friday morning, his wife said.

Mr Yu has called for open presidential elections and constitutional reform.

Hundreds of human rights lawyers have been detained and interrogated by authorities in recent years.

Hours before his detention, Mr Yu criticised China's leadership and called for political reform in an open letter (in Chinese).

"Designating the nation's president, as head of state, through a single party election has no meaning as an election," he said.

"It has no power to win confidence from the nation, civil society, or the world's various countries."

It is not clear where Mr Yu is being held, and local police told the AFP news agency they were not aware of his detention.

His wife, Xu Yan, told Reuters news agency: "I have not received any legal document about his detention and don't even know what crime he is suspected of committing."

Mr Yu revealed that his licence to practice law had been revoked earlier this week.

Officials said he had not been employed by a licensed legal firm in the past six months, but human rights activists alleged that it was the result of his vocal opposition to the ruling Communist Party leadership.

Amnesty International's China researcher, Patrick Poon, told AFP: "It's likely retaliation against him for talking to media."

"I'm worried he might be charged with a serious offence like 'inciting subversion of state power' for his words," he added.

Mr Yu, a former commercial lawyer, has worked on many sensitive cases, including defending other human rights lawyers detained by the government.

Image copyright Getty Images Image caption Chinese President Xi Jinping cemented his political power in October's party congress

He is also part of the team that tried to sue the Chinese government over its failure to improve air quality in the capital, Beijing.

More than 300 lawyers, legal assistants and activists have been questioned, and more than two dozen have been formally investigated since a government-led crackdown that began in 2015.

Some have been sentenced to long jail terms, while others are still awaiting a sentence.