BEIJING — One of China’s most prominent human rights activists, the blogger Huang Qi, went on trial on Monday on charges of leaking state secrets, and American diplomats seeking to attend the proceedings said they had been denied access.

The trial has drawn intense international attention, in part because Mr. Huang, 55, has nephritis, a potentially fatal kidney disease. In addition, Mr. Huang’s 85-year-old mother recently traveled from their home in Sichuan Province to Beijing to ask foreign embassies there for help in obtaining his release, but she was reportedly beaten and is now detained in a hospital.

American diplomats seeking to attend the trial in the western Chinese city of Mianyang were “denied access to the courtroom,” a press officer for the United States Embassy said on Monday.

Mr. Huang founded the website 64 Tianwang, which relied on a nationwide network of volunteers to report on land expropriations, corruption and labor disputes. The name refers to the June 4, 1989, crackdown on protesters around Tiananmen Square in Beijing. The site’s core mission, it says, is to “stand in solidarity with those who have no power, no money and no influence.”