The National People's Congress' nearly 3,000 hand-picked delegates endorsed the constitutional amendment on Sunday, voting 2,958 in favour with two opposed, three abstaining and one vote invalidated.
It means President Xi Jinping can rule indefinitely.
Benedict Rogers, East Asia team leader at religious freedom charity Christian Solidarity Worldwide (CSW) said the vote is an "extremely significant development, which will in all likelihood mean a continuing crackdown on human rights generally, and religious freedom in particular".
He added: "The past five years since Xi Jinping became president have seen the most severe and widespread crackdown on human rights since the Tiananmen massacre of 1989.
"In particular we have seen the arrest and disappearance of human rights lawyers, many of whom are Christians or have been involved in defending religious freedom, and increasing restrictions on religious freedom, including the destruction of thousands of crosses in Zhejiang province, the destruction of several churches, increasing persecution of other religious groups such as Uighur Muslims, Tibetan Buddhists and Falun Gong, and the introduction of new regulations tightening rules on religious practice."
The amendment overturns a system enacted by former Chinese leader Deng Xiaoping in 1982 to prevent a return to the bloody excesses of a lifelong dictatorship typified by Mao Zedong's chaotic 1966-1976 Cultural Revolution.
The constitution had until now limited presidents to serving only two consecutive terms.
Rodgers told Premier that now Xi Jinping is President for life and his power-base is strengthened, Christians in China can expect to see "continuing, perhaps increasing, repression".
He added: "We also expect to see China to use its increasing power to act to prevent progress on human rights around the world. These are very dark days for China – with very serious implications for the world."
A number of prominent Chinese figures have publicly protested against the move, despite the risk of official retaliation.
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