SHANGHAI - Basketball followers on mainland China expressed anger at the U.S. National Basketball Association on Wednesday after a fan event in Shanghai was cancelled, amid a row over free speech that has hurt the league's reputation in a key overseas market.

Chinese organisers scrapped the event on the eve of a planned exhibition game between the Los Angeles Lakers and Brooklyn Nets, as the fallout spread from a tweet by Houston Rockets general manager Daryl Morey supporting recent protests in Hong Kong.

Pro-democracy demonstrators have taken to the streets of the city over fears that Beijing is undermining Hong Kong's "one country, two systems" formula, which gives residents greater freedom than mainland Chinese.

China has warned foreign governments to steer clear of the issue which they deem an "internal affair".

NBA Commissioner Adam Silver said on Tuesday that the league supported Morey's right to exercise his freedom of expression, furthering angering Chinese authorities.

"They (the NBA) have done something really stupid," said Bai Linfan, a 20-year-old fan in Shanghai. "This is something that is really about our nationality. That's the biggest thing. Cancelling these cooperations and exchanges is quite normal."

Another young fan, Li Xintong, said the NBA needed to do more to understand China's view on the matter.

"If the pre-season match is cancelled, I don't really care, because I think this thing has already violated our country's bottom line about a few issues," he said.

"If the NBA really wants to expand in China's market, they must understand Chinese culture."

The reaction to the row has been very different in Hong Kong itself.

"I don't think he (Silver) is mixing up sports and politics. I think he was speaking as a U.S. citizen on this occasion. I'm really grateful for his support and the support of the majority of U.S. citizens," said NBA fan Keith Yuen.

For one fan hoping to catch a glimpse of the players in Shanghai, sport and politics should not be mixed.

"I don't know why political conflicts affect sporting events. Just because they don't want us to see Morey's comments on TV, the fans' night has been cancelled," said Yu Jie.

"I'm patriotic of course. I support (the view) that Hong Kong is part of China, but I just don't understand this."

(Reporting by Xihao Jiang and David Stanway; Writing by Hugh Lawson;) ((hugh.lawson@thomsonreuters.com; +44 207 542-8384))