By curtailing the trade, China, which has been criticized for not properly enforcing earlier sanctions, is obeying the intent of the latest sanctions resolution but harming its own businessmen.

“I think it is very likely that I need to return my truckload of seafood back to North Korea — and what’s worse, they won’t give my money back,” Zhang Xuebai, a wholesale trader, said in a telephone interview. “I will probably lose about $45,000. For other businessmen who have more goods stuck there, they can lose $150,000.”

The Chinese traders typically do not keep close track of international relations, and although they know that ties between China and its wayward ally, North Korea, can be touchy, they were not given any warning about the sanctions enforcement, Mr. Zhang said.

“This just happened out of the blue,” he said.

The red banners held aloft by protesters expressed the traders’ anger at the Chinese government.