"But I wouldn't trade it for anything in the world."

Mrs Obama, who arrived in China on Thursday evening, avoided directly criticising Beijing's draconian control of the internet, media and religion.

Social media sites including Facebook, YouTube and Twitter are blocked in China and Xi Jinping, the president, has been waging a fierce war on dissent since coming to power in November 2012.

Xu Zhiyong, a lawyer and activist behind a peaceful campaigning group called the New Citizens' Movement, was jailed for four years in January for "disrupting public order."

Earlier this month, Cao Shunli, a 52-year-old activist, died after falling into a coma while in police custody. Ms Cao, who died of apparent organ failure, had been taken into custody last September as she tried to fly out of Beijing to a human rights workshop in Geneva.

Campaigners also criticise China's handling of religion, with non-official "underground" churches banned and restrictions placed on freedom of worship, particularly in regions such as Xinjiang, which is home to a large Muslim community.

China's heavily controlled state media made no mention of Mrs Obama's comments.

The US First Lady's speech "focused on the importance of education and cultural exchanges," state broadcaster CCTV reported. "She said China is currently the fifth most popular destination for American students abroad."

Students had given Mrs Obama "a warm welcome," CCTV added.

Mrs Obama arrived in China on Thursday night and on Friday toured its capital with Peng Liyuan, the first lady, and met Xi Jinping at the Diaoyutai State Guesthouse.

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Freedom of speech is 'universal' right, Michelle Obama tells China