According to his indictment, Pu had made seven comments on the Twitter-like Weibo between July 2011 and May 2014 -- three criticized government officials and a pro-government author, while the rest were critical of Chinese policy in ethnically divided Tibet and Xinjiang.
Supporters and journalists scuffled with public security officials outside the courtroom and video posted on Twitter showed U.S. diplomat Dan Biers being shoved as he read a statement.
The U.S. Embassy in Beijing said it viewed "with great concern any incident in which diplomats are not accorded the protection and respect consistent with their status."
In a separate statement, it urged China to release Pu, describing the charges against him as vague.
"Lawyers and civil society leaders such as Mr. Pu should not be subject to continuing repression, but should be allowed to contribute to the building of a prosperous and stable China."
Searches for Pu's name were blocked on Weibo. On WeChat, a popular messaging service similar to Facebook, some users shared posts in support of the lawyer, while others switched their profile pictures to an image of Pu.
Pu, 49, was detained in May 2014 and formally arrested the following month after attending a low-key seminar in a private home to mark the 25th anniversary of the Tiananmen Square massacre.
The state-run Global Times newspaper said in an editorial at the time that he had crossed "a legal red line" by associating himself with a topic still considered taboo in China.
Pu took part in the student-led demonstrations in 1989 that ended in a bloody military crackdown on June 4 of that year. He later become one of the best-known lawyers in China for defending human rights in courts as well as in the media.
Although his work has often put him at odds with the ruling Communist Party, Pu dismissed the risks in a 2013 interview with CNN.
"I think I'm fine," he said. "I'm a moderate, and the government has treated me well. I'm a veteran lawyer and haven't made mistakes in my career. I'm not radical, and I don't threaten the government."
Human Rights Watch said that Pu was suffering from diabetes but authorities denied a request to have him released on medical grounds.