Vietnamese activists took to the streets in protest against China this week in a rare demonstration in the Communist nation as tensions flare over rival claims on the oil and gas-rich South China Sea.

Authorities initially allowed about 200 protesters, carrying banners with “Down with China’s aggression!”, but their march was intercepted within half an hour as they headed towards the Chinese embassy in the capital Hanoi.

Police urged the crowd to disperse and reassured them, saying: “The Communist Party and government are resolutely determined to defend our country’s sovereignty and territory through peaceful means based on international law. Your gathering causes disorder and affects the party’s and government’s foreign policy.”

Around 20 activists were rounded up into a bus and taken to a detention center on the outskirts of the city after they some attempted to continue the demonstration, AFP reports.

Hundreds of activists took part in similar protests in Ho Chi Minh but these were also broken up by security forces.

Tensions between the Communist neighbors heightened after state-owned oil and gas company PetroVietnam accused Chinese boats of sabotaging its exploration cables in the South China Sea.

Beijing denied the allegations and demanded Vietnam halt oil exploration in the waters. China claims virtually all of the South China Sea and has wracked the region with its growing military assertiveness.