Chinese prosecutors say they have indicted a human rights lawyer and three activists on subversion charges, a sign that an unprecedented crackdown on legal activism that began one year ago continues despite drawing international criticism.

The prosecutors' office of the municipality of Tianjin said in a posting on its official microblog Friday that Zhou Shifeng, a lawyer who heads a Beijing-based firm seen as being at the center of the security sweep, has been charged with subversion of state power.

The vaguely defined charge carries a maximum sentence of life imprisonment.

The prosecutors said three activists were indicted on the same charge.

Rights groups say the activists are being targeted for organizing protests and social media campaigns to raise awareness of legal rights and hot-button social issues.