NEW DELHI— The detention of Pema Wangjom Thongdok, an Indian citizen living in the UK, at Shanghai airport has fuelled a diplomatic row between Delhi and Beijing.

Ms Thongdok said she was detained by immigration officials while travelling from London to Japan via Shanghai last Friday because her passport lists her birthplace as the northeastern state of Arunachal Pradesh.

China disputes India's claim over the region and considers Arunachal its territory, calling it "South Tibet" — a claim India firmly rejects.

On Tuesday, India said it had lodged a protest against Ms Thongdok's detention and reiterated that Arunachal was an "integral and inalienable" part of India.

Ms Thongdok, who called the experience "humiliating", contacted the Indian embassies in Shanghai and Beijing for help. She was released from detention after 18 hours when Indian officials arrived at the airport and spoke to the authorities.

India's foreign ministry spokesperson Randhir Jaiswal called Ms Thongdok's detention "arbitary" and said the issued had been taken up strongly with China.

"Chinese authorities have still not been able to explain their actions, which are in violation of several conventions governing international air travel. Their actions also violate their own regulations that allow visa free transit up to 24 hours for nationals of all countries," he said in a statement.

Jaiswal's statement came hours after China's foreign ministry spokesperson Mao Ning said that China's border inspection authorities had "carried out checks in accordance with laws and regulations".

"The law enforcement was impartial and non-abusive, the lawful rights of the individual [Ms Thongdok] were fully protected, no compulsory measures were taken on her, and there was no so-called 'detaining' or 'harassing'," she told India's PTI news agency. She added that the airline — China Eastern — had provided Ms Thongdok resting facilities and meals.

But she insisted that Arunachal was China's territory.

"The Chinese side has never recognised the so-called 'Arunachal Pradesh' illegally set up by India," she said.

The state, which falls along a 3,440km (2,100 mile) de-facto border in the Himalayas — called the Line of Actual Control — has been a source of tension between the Asian neighbours for decades.

China says it considers the whole of Arunachal Pradesh its territory, while India claims the Aksai Chin plateau in the Himalayas, which is controlled by China.

Troops from the countries clashed along the border in the town of Tawang in Arunachal Pradesh in 2022. The fight took place even as India and China were working to re-establish ties after a clash at the Galwan Valley in the Ladakh region in 2020 led to a complete breakdown in ties.

Over the past year or so, India and China have taken some big steps towards normalising relations, including re-starting airplane services to each other's countries and agreeing on patrolling arrangements to de-escalate border tensions.

In August, Chinese Foreign Minister Wang Yi, who was in Delhi on a two-day trip, said the neighbours should view each other as "partners" rather than "adversaries or threats".

So, Ms Thongdok's detention amidst these friendly overtures has raised eyebrows in India and made national headlines.

She told ANI news agency that she had been living in the UK for 14 years and had travelled through Shanghai several times earlier without facing any issues.

However, this time an immigration official at the airport "singled" her out of the queue and told her that her passport was invalid, she said.

"When I tried to question them and ask them what the issue was, they said, 'Arunachal is not part of India' and started mocking and laughing and saying things like 'you should apply for the Chinese passport, you're Chinese, you're not Indian'," she added. — BBC