SHANGHAI (Reuters) - When the Costa Serena cruise ship steamed into the South Korean tourist island of Jeju, Bai Liyun stayed on board, where crew had organized a magic show, games and even a wine-tasting.

Cruise Costa Serena carrying Chinese passengers who refused last week to disembark on the resort island of Jeju, is seen back at Wusongkou port in Shanghai, China, March 14, 2017. REUTERS/Aly Song

She was not alone.

Most of her Chinese shipmates, estimated by passengers to number more than 3,000, followed suit, in a show of solidarity with their government’s vociferous opposition to South Korea’s decision to deploy a controversial missile defense system.

“We spent the whole day on the cruise ship. I felt very happy,” Bai, from China’s western province of Gansu, said on Tuesday in Shanghai, where the passengers disembarked on their return.

“As Chinese, we surely should answer the government’s call at a special time, which meant not going to Jeju Island.”

But the decision meant tour guides and about 80 tour buses were left waiting at the port, South Korean media reported.

China has publicly opposed the deployment of the Terminal High Altitude Area Defense (THAAD) missile system, calling its powerful radar a threat to the country’s security.

However, South Korea and the United States say the system is aimed solely at defending the South against a growing North Korean missile threat.

Nevertheless, Beijing has stepped up pressure on companies doing business with, and in, South Korea, although it has not directly said it was targeting South Korean firms in retaliation.

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The stakes are high for South Korea, and tourism is a particularly vulnerable sector. South Korean data show almost half of its visitors come from China.

Last week, airlines such as China Eastern Airlines Corp Ltd and Spring Airlines Co Ltd stopped website offers of flights between China’s eastern city of Ningbo and Jeju.

The decision to boycott Jeju “was made on mutual consensus” among the cruise passengers, most of whom were employees of the Chinese direct-sales company Resgreen Group on a junket, said a company spokeswoman, Jiang Wen.

The company would soon issue a statement, another spokeswoman added.

Bai, a Resgreen employee, said colleagues had sent round messages before the trip to suggest that nobody set foot in South Korea. The company made the decision official before the cruise docked at Jeju, she said.

While South Korean exports from mascara to music have grown wildly popular in China, the port boycott shows that patriotism, deftly stoked by the ruling Communist Party, can quickly be brought to bear in diplomatic tiffs.

Bai’s cruise partner and relative, Bai Lijuan, said the decision was the right one.

“(THAAD) is a threat to Chinese,” she said. “We must be resolutely opposed.”

A passenger from southern Hunan province who stepped off the boat in Shanghai said passengers had been told not to disembark in Jeju, but he was ambivalent about the missed South Korean port call.

“The trip was all about shopping. No sightseeing, just shopping. That’s a lot of spending,” said the passenger, in his early twenties, who did not give his name.

“At least I didn’t need to spend anything in Jeju.”