Yu Wensheng, 50, was charged last weekend with "inciting subversion of state power", a week after he was detained as he prepared to take his son to school in Beijing.
Best known for suing the Beijing government over the city's once chronic pollution, Yu has been a persistent voice for reform despite an increasingly severe crackdown on activism under President Xi Jinping.
Hours before his detention, he had circulated an open letter calling for five reforms to China's constitution, including the institution of multi-candidate presidential elections.
"All Yu Wensheng has done is campaign for democratic reforms in China and support fellow citizens who were harassed for exercising their human rights," she said, calling for his release "without delay".
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Chinese foreign ministry spokeswoman Hua Chunying responded today that China was "a country of rule of law" and that those who break it must "shoulder the responsibility" of their actions.
"We firmly oppose any foreign government, country or individual interfering in China's internal affairs. I don't think such an official ... has the right to ask China to release anybody," she told a regular press briefing.
Yu had said that in 2014 authorities imprisoned and tortured him for 99 days for allegedly "disturbing public order".
For several days beginning on July 9, 2015, more than 200 Chinese human rights lawyers and activists were detained or questioned in a police sweep that rights groups called "unprecedented".
The "709 crackdown," as it was later dubbed, marked the largest clampdown on the legal profession in recent history.
Yu was not arrested during the sweep and had continued to express his opinions on legal issues.