By Michael Bristow
BBC News, Beijing
A prominent Chinese activist has been formally arrested more than a month after being taken into custody.
Hu Jia was arrested in his dining room, two days after Christmas
Hu Jia, who publicises human rights abuses across China, has been accused of inciting subversion of state power.
Campaigners say his arrest shows that China is not keeping its promise to improve human rights ahead of this year's Beijing Olympic Games.
But the government says China is a country ruled by laws, and Hu Jia will be dealt with according to the law.
Two days after Christmas, about 30 security officers burst into Hu Jia's flat and took him away.
Officials were apparently tired of his efforts to support human rights cases across the country.
He had become a kind of one-man clearing house for information, passing it on to journalists, organisations and foreign embassies.
His wife Zeng Jinyan, also a prominent activist, has been put under house arrest with the couple's two-month-old baby.
The BBC was not allowed to visit her when we went to the couple's flat on the outskirts of Beijing last month.
Since he was detained, there has been little word about Hu Jia's condition and whereabouts. His lawyers have not been allowed to see him.
But officials have now sent his family formal notification that he has been arrested.
"After 37 days a suspect has to be either released or formally arrested. Hu Jia has been arrested," said a family friend, lawyer Teng Biao.
Mr Hu faces charges of subverting state power, a serious charge that could lead to a long prison term.
Rights groups say Beijing is cracking down on dissent ahead of the Olympics in August.
"The preparations for the Olympics are having an overall negative impact on human rights developments in China," said Sophie Richardson of Human Rights Watch.
The United States and the European Union have also voiced their concern about Hu Jia's arrest. But so far, this criticism has had no visible effect.