Dozens of human rights groups have signed a joint petition calling on Laos authorities to release a prominent Chinese lawyer as its embassy in Britain warned he could be deported back to China.

Concerns for Lu Siwei have spiked after he went missing in Laos while traveling to the United States. Laos lies across China's southwestern border and has long been a common, albeit risky, exit point for Chinese dissidents trying to leave the country.

The 85 rights groups from around the world demanded the Laotian government "halt all processes of repatriation for Lu Siwei and release him immediately according to its international human rights obligations."

"We are gravely concerned that he is at serious risk of forced repatriation to China where he faces the high likelihood of torture and other ill-treatment," it said, also calling for the country to disclose his whereabouts and safety.

CNN was unable to reach Laos authorities on Friday, but its embassy in Britain confirmed his arrest later that day to the UK-based lawyers group, The 29 Principles.

It said Lu was arrested on July 28 "on suspicion of using fraudulent travel documents" but added that he was "healthy", according to an email seen by CNN.

"He is now waiting for investigation and criminal proceedings that will take place soon according to the laws of Lao PDR (Lao People's Democratic Republic," the embassy said, adding: "If found guilty, Mr. Siwei will be deported in accordance with the laws and regulations of the Lao PDR and treaties the Lao PDR is party to."

Human rights groups said he has a valid visa to enter the country.

Lu, from the southwestern Chinese province of Sichuan, has represented a long list of defendants considered dissidents in mainland China over the years.

He was known for navigating China's opaque criminal system until his license was revoked by authorities in 2021 following years of taking on sensitive cases.

The signatories of the joint petition said Lu has been "closely monitored by the Chinese authorities and subject to an exit ban since May 2021."

Being a human rights lawyer has become increasingly risky under Xi Jinping, China's most assertive leader in a generation.

Hundreds of well known rights lawyers were arrested in 2015 in what was later dubbed the "709 crackdown."

Lu represented fellow human rights lawyer Yu Wensheng after he ended up in the dock for taking on the cause of a number of those detained in the initial roundup.

More recently Lu lost his practicing license after representing one of the 12 activists who were intercepted by Chinese coastguards at sea on their way to Taiwan while trying to flee Hong Kong.

Detention witnessed

US-based group ChinaAid, which describes itself as an "international non-profit Christian human rights organization" and is one of the petition signatories, said its activists witnessed Lu's detention in Laos. The group said Lu had obtained visas for both Laos and the US and was on his way to be "reunited with his family."

"Immigration police apprehended (Lu) in front of two staff members of ChinaAid. There has been little information about Lu since his arrest on July 28th," the group said in a statement.

The group posted video on YouTube of what they said was the moment Lu was picked up. It showed three men in uniform and a man who looks like Lu next to a white Toyota car on the side of the road.

They told CNN that an earlier version of the petition with 75 signatories was passed to the Laos embassy in Britain earlier, with a protest planned in London on Saturday.

Spokesperson Sam Yip, of Japan Hong Kong Democracy Alliance, which also signed the petition, described how the groups are racing against time given the Laos government's "negative track record" in how it treats activists domestically.

"There's a real emergency to have him stopped from being extradited back to China to protect his own security," he told CNN.

China has a long history of imposing exit bans on individuals in an attempt to force suspected criminals to surrender, to resolve business disputes, or maintain pressure on political dissidents and regime critics.

Laos is a popular route for Chinese trying to avoid legal emigration. Many head on to Thailand, a major hub for international flights.

But the Laos route carries dangers.

Officially known as the Lao People's Democratic Republic, the country has one of the world's few remaining communist governments, which runs under an opaque one-party system whose leaders have close ties with Beijing.