SEOUL — Protesters angry over American demands that South Korea pay more for defense destroyed portraits of the U.S. ambassador stuck on blocks of tofu outside the American embassy Friday, after police warned them against staging a more aggressive demonstration.

U.S. Ambassador Harry Harris has become a political lightning rod for South Koreans angered by President Donald Trump's push to get South Korea to pay billions of dollars more toward maintaining the 28,500 American troops stationed there.

"Harris out! We are not a U.S. colony! We are not an ATM machine!" the demonstrators chanted outside the embassy, surrounded by phalanxes of police.

The left-leaning protesters from several youth groups cheered as two students smashed up blocks of tofu and acorn jelly adorned with paper portraits of Harris.

Longtime allies, the United States and South Korea are in dispute over how much each should pay for the U.S. troops in South Korea. Trump has demanded Seoul pay as much as $4 billion more a year, according to South Korean officials, and a new round of talks is scheduled in Seoul next week.

South Korea currently contributes about $900 million to the upkeep of U.S. troops in the country.

"How is it possible for a tenant to ask for five-fold increase in rent from its landlord?" Kwon Oh-min, a Youth Party representative, said outside the U.S. embassy on Friday.

South Korea has the third-largest presence of U.S. troops based overseas after Japan and German. Major bases include the U.S. Army Garrison Humphreys in Pyeongtaek, south of Seoul, which covers 14.7 million square meters.

In October, dozens of progressive university students climbed into the grounds of the ambassador's residence in protest against the U.S. presence in the country, sparking calls from the embassy for more police protection.

Another gathering of progressive groups is planned for Saturday outside the U.S. embassy to urge Washington to withdraw its request for defence cost-sharing increases ahead of the next round of the Special Measures Agreement (SMA) talks next week.

Seoul police told Reuters they had warned Friday's protesters not to bring hazardous materials or propaganda that could be construed as defamatory or insulting and that they planned to "restrict the protests if they cross the line".

The groups had initially planned to behead an effigy of Harris but scaled back their demonstration to smashing up the blocks of traditional Korean food after the police warning.

Across the street from the U.S. embassy, a rival gathering of conservative, pro-Washington protesters hit people wearing the masks of North Korean leader Kim Jong Un and South Korean President Moon Jae-in with toy hammers.