Chinese authorities detained a prominent human rights lawyer on Friday, people familiar with the case said, just hours after he provided journalists with a letter calling for constitutional reform.
Around a dozen people, including a SWAT team, seized Yu Wensheng as he left his Beijing apartment to walk his child to school, two sources told AFP.
Local police said they were unaware of his detention.
Yu has been a persistent voice for reform in China, despite the country's sweeping and increasingly severe crackdown on civil society under President Xi Jinping, which has led to the jailing of numerous human rights litigators.
Just hours before Yu's detention, he had circulated an open letter calling for five reforms to China's constitution, including the institution of multi-candidate presidential elections.
"Designating the nation's president, as head of state, through a single party election has no meaning as an election," he wrote.
"It has no power to win confidence from the nation, civil society, or the world's various countries."
The issue has always been a sensitive one in China, but has become even more so in recent years, as Xi's rise to the position of the country's most powerful leader in a generation has been accompanied by stern warnings against questioning his position as the Communist Party's "core".
- 'Retaliation' -
Yu has defended prominent civil rights lawyers targeted by the government and people detained for supporting Hong Kong's pro-democracy movement.
Yu has said that in 2014 authorities imprisoned and tortured him for 99 days for allegedly "disturbing public order".
He is perhaps best known for being one of six lawyers who attempted to sue the Chinese government over the country's chronic smog.
At the time, he scoffed at concerns that he might be detained for his actions, telling AFP last year: "If we do things according to the law and still get detained... it will be just the thing to show people the true nature of our so-called 'rule of law'."
During the Communist Party congress in October, during which Xi further consolidated power, Yu penned an open letter to delegates saying "China has no freedom, no democracy, no equality, no rule of law".
Yu said he was later interrogated by police for three hours.
He was recently suspended from practice and his application for starting a new law firm was also rejected, according to human rights organisation Amnesty International.
"It's likely retaliation against him for talking to media," the group's China researcher Patrick Poon told AFP.
"I'm worried he might be charged with a serious offence like 'inciting subversion of state power' for his words."
In December, China imprisoned activist Wu Gan for eight years on the charge in what many experts considered an unusually severe punishment.
Xi has increasingly stifled civil society since taking office in 2012, targeting everyone from activists to human rights lawyers and teachers to celebrity gossip bloggers.
More than 200 Chinese human rights lawyers and activists were detained or questioned in a police sweep in 2015 that rights groups called "unprecedented".
Last year, democracy activist and Nobel peace prize laureate Liu Xiaobo died of liver cancer while still in custody as authorities rejected his request to seek treatment abroad.
A veteran of the 1989 Tiananmen Square protests, Liu was sentenced to 11 years in jail in 2009 for "subversion" after pushing for democratic reforms.