The Philippine embassy in China has warned its citizens to beware of personal "threats" and avoid political debates ahead of a tribunal ruling Tuesday over a bitter dispute in the South China Sea.

Manila filed a case with an international tribunal in The Hague in 2013 challenging China, which claims nearly all of the strategically vital sea, even waters approaching the coasts of the Philippines and other Southeast Asian nations.

China -- whose governing Communist party uses nationalism as part of its claim to a right to rule -- refused to participate in the hearings and has vowed to ignore Tuesday's decision.

In an email to Philippine citizens in China, the embassy warned them to be "careful" because of tensions ahead of the arbitration.

They were advised to "avoid meetings and public discussions on political issues" and discouraged from joining political discussions or debates "especially on social media networks", according to the email seen by AFP.

They were urged to carry identification papers "at all times" and report any threat received to the embassy and Chinese police.

Nationalist protests are not rare in China, sometimes apparently with the tacit backing of authorities.

Chinese protesters took to the streets in major cities after Tokyo in 2012 nationalised some of the disputed islands known as Diaoyu in China and Senkaku in Japan in the East China Sea.

They attacked Japanese diplomatic facilities and businesses, harassed individual Japanese and turned over Japanese-brand vehicles in demonstrations initially condoned by authorities, who eventually restricted them.

More than 20 Chinese police were positioned outside the Philippine embassy on Tuesday morning, with more in vans nearby -- a significantly larger presence than usual -- along with two lorries loaded with crowd control barriers, a possible indication that authorities expected protests at the building.