Vietnamese police broke up anti-China protests in two cities on Sunday and detained 20 people in the first such demonstrations since tensions between the neighbours flared anew over rival claims to the oil and gas-rich South China Sea. Any sign of popular anger in tightly controlled Vietnam causes unease among the leadership, but anti-Chinese sentiment is especially sensitive. The country has long-standing ideological and economic ties with its giant neighbour, but many of those criticising China are also the ones calling for political, religious and social freedoms at home.

Police initially allowed about 200 protesters to march from the Opera House in Hanoi through the streets, but after 30 minutes ordered them to disperse. When some continued, police arrested about 20 demonstrators and put them into a large bus that then quickly drove from the scene. It was unclear where they were taken, but in the past people detained at anti-China protests have been held briefly then released.

As tourists and morning strollers looked on, protesters shouted “Down with China!” and waved banners bearing the slogan “China’s military expansion threatens world peace and security.”

A smaller protest also took place in Ho Chi Minh City, according to blogger and activist Huynh Ngoc Chenh. He about 100 people gathered for 10 minutes before being dispersed.

Vietnam has alleged that Chinese shipping vessels sabotaged one of its seismic survey vessels in the South China Sea. China had earlier issued new passports containing a map showing the sea as belonging to it.