Beijing, China (CNN) -- Japanese in China are on edge heading toward the weekend amid outrage over Japan's detention of a Chinese fishing captain.
The Japanese Embassy in China has warned Japanese citizens living there to be on guard in crowds, and at Japanese-themed sites such as restaurants and hotels. Massive anti-Japanese protests are expected in Beijing, Shanghai and Shenyang on Saturday.
The captain was arrested last week off the disputed Diaoyu Island in the East China Sea after his vessel crashed into two Japanese patrol boats. He remains in custody, accused of obstructing Japanese public officers while they performed duties.
Japan freed his 14 crew members, who have flown back to China.
On Wednesday morning, their fishing boat returned from Japan. A replacement Chinese crew sailed the vessel to the city of Quanzhou in China's eastern Fujian province under escort by two Chinese fishery administration ships.
With Saturday's protests brewing, China's Public Security Bureau has issued a statement about boosting public security. The government has final say over protests, and could pull the plug on them. But there was no such indication on Thursday.
"At present, Japan is still illegally detaining the Chinese boat captain. It is the most pressing obstacle in China-Japan relations," said Chinese Ministry of Foreign Affairs spokeswoman Jiang Yu. "We hope they will take immediate actions to eliminate the obstacles as soon as possible."
Word of the fishermen's arrest has led to irate reactions from some of the Chinese public, as seen in online posts.
On Sunday, a Japanese school in northeastern China was fired on with small BBs, presumably in connection with the fishermen's arrest.
China has repeatedly summoned the Japanese ambassador, demanding the captain's release.
Beijing says the Diaoyu Islands and most of the South China Sea belong to China, disputing neighboring countries' claims. The clash over territorial waters and islands -- and the natural resources that go with them -- is a flash point in the Asia-Pacific region.
"Chinese people have expressed strong indignation to this, and this fully embodies the Chinese people's resolve to defend national sovereignty," Yu said earlier this week. "The islands has sovereign rights, and no one can change that fact."
"If you read history documents, Chinese first discovered the island and put it in effective administration," Yu said.
CNN's Jo Ling Kent contributed to this report.