A Chinese activist was detained in South Korea after crossing more than 300km of open sea in a rubber boat, spending nearly 30 hours sailing in a fourth attempt to flee the country and reunite with his family in Canada.

Dong Guangping, 68, a former police officer who was previously jailed for his activism, was spotted 38 nautical miles off the South Korean coast on Monday evening by a fishing boat, which then alerted the coast guard.

He had left from China's eastern Shandong province on a 3.3-metre boat with a 10 horsepower motor.

In a statement on Wednesday, the South Korean coast guard confirmed that a Chinese man in his 60s had been arrested and was being questioned on suspicion of immigration law violations, according to Reuters.

Mr Dong was fired from the police force in 1999 after co-signing a letter marking the 10th anniversary of the Tiananmen Square crackdown on pro-democracy protests. He was then imprisoned for three years in 2001 for "inciting subversion of state power" and arrested again in 2014 for participating in another Tiananmen commemoration.

This is at least his fourth attempt to leave China. In 2015, he fled to Thailand with his wife and daughter, where the family was granted refugee status by the UN. While his wife and daughter were able to travel to Canada, Mr Dong was deported back to China. He was sentenced to three-and-a-half years in prison and released in 2019.

That same year, he tried to swim to Kinmen, an island controlled by Taiwan a few kilometres off China's eastern coast, but was reportedly picked up by Chinese fishermen and handed to police.

In 2020, he crossed into Vietnam and lived there for two years before being detained and deported. He was released in October 2023 after serving 11 months for illegal border crossing.

“For more than a decade, he has never ceased striving for liberty and reunion with his family," Human Rights in China, a rights group in New York, said in a statement. "That a man nearing seventy years old was driven to cross open seas in a small inflatable boat is itself a devastating indictment of the human rights situation in China."

The group urged Seoul to uphold humanitarian principles and not return Mr Dong to China, where it said he faced “a grave risk of persecution and torture”.

South Korea and China have had an extradition treaty in force since 2002, but it does not obligate Seoul to extradite in all cases.

Mr Dong is not the first Chinese dissident to attempt the crossing. Another activist, Kwon Pyong, made a similar journey by jet ski in 2023 and was held by South Korean authorities for several months before eventually resettling in the United States.