Story highlights Li Baiguang represented Chinese Christians and other religious minorities

He met with US President George W Bush and received an award from the National Endowment for Democracy

Hong Kong (CNN) A prominent Chinese lawyer, Li Baiguang died Monday in the Chinese city of Nanjing under "mysterious" circumstances, according to Bob Fu, a US-based activist and Christian pastor who has known the lawyer for over 10 years.

Li was admitted to the No. 81 Military Hospital with a minor stomach ache, but had been otherwise healthy, Fu said, citing a relative of Li's. He was declared dead hours later from liver complications, according to the activist.

Critics allege Li has fallen victim to the rights crackdown which has characterized the rule of President Xi Jinping , just as the Chinese leader seeks to extend his grip on power.

On Sunday, Chinese state media said the country's constitution would be amended to drop term limits on the Presidency, clearing the way for Xi to lead the country indefinitely.

The move has alarmed critics who say that Xi's tightening grip on power has come at an increasing cost to human rights and those who seek to defend them.