“I was originally worried that I would be arrested at the airport. But so far, everything has gone all right,” he said at the airport in Beijing on Sunday before leaving for the United States. He later confirmed his arrival at Newark by email.

“I don’t know whether I’ll have any problems when I try to return to China.”

He added that he may take positions with Wellesley College, Harvard University or at a think tank in Washington.

Outspoken figures have previously been prevented from boarding international flights, including two activists preparing last year to participate in a United Nations review of human rights, and Uighur academic Ilham Tohti, a critic of government policy towards the minority, who was detained this month.

Xia’s sacking drew criticism from professors overseas, who argued that China’s already limited academic freedom was at stake.

Speaking last month after finishing his final class, he said: “I’ve been teaching at Peking University for more than a decade. No student has ever told me I’m a terrible teacher.”

China’s ruling Communist Party takes a hard line against dissenters who might threaten its power, and new leaders under party chief and President Xi Jinping have cracked down on activists since taking office about a year ago.

Several activists have gone on trial in recent days, including Xu Zhiyong, a prominent legal campaigner and central figure in the New Citizens Movement seeking to uphold individual rights, who was sentenced to four years in prison.

Xia said the sentence “makes completely no sense”.

Liu Xiaobo, who co-authored Charter 08 and won the 2010 Nobel peace prize, is serving an 11-year jail sentence.