Shanghai —

Michael Phelps recently tweeted his mother from the American team’s training camp in Australia. The 14-time Olympic champion might have to resort to more conventional modes of conversation when he arrives this week at the world championships in China, where Twitter and Facebook are blocked by the government.

Unless users know how to create their own virtual private network (VPN) or are willing to spend heavily on roaming fees and post from foreign-based smart phones, access to foreign social-networking sites in China is difficult.

The government began blocking foreign social-networking sites in the first half of 2009 and shut down some Chinese clones after deadly ethnic riots in China’s heavily Muslim Central Asian border region of Xinjiang.

“We haven’t even tried. We’ve been to China so many times that we just know you can’t do it,” said Australian diver Sharleen Stratton after pairing with Anabelle Smith to win the bronze medal in 3-meter springboard synchro on the first night of competition Saturday.

“So we kind of prepare ourselves before we go and tell everybody they won’t hear from us for three weeks.”

Competing in his first world championships, American diver Aaron Fleshner wasn’t completely prepared for the news.

“I use social media a lot when I’m at home. So with everything blocked here, I can’t get a hold of anybody,” he said.

“I’ve got a lot of free time now. I’m reading books now,” added Fleshner, who began a fantasy-adventure novel, “The Lightning Thief,” on his way to the pool Saturday.

Facebook was available in China when Phelps won a record eight golds at the 2008 Beijing Olympics, and Phelps recently started posting on Twitter – his following has quickly shot up to 56,000 followers.

On Friday, Phelps posted this message to his mother, Debbie, from his training base on Australia’s Gold Coast: “MamaPhelpsH20 thanks mom!!! Miss u and love u!!! See u when u get over here!”

China had committed to a more open Internet during the 2008 Olympics, at least in Beijing. But that was before the Iranian protest movement of 2009 and this year’s “Arab Spring,” when social networks proved their capabilities for organizing protest rallies against governments.

Some Chinese circumvent controls by using VPN services, though that has become more difficult in recent months. At the same time, the popularity of a homegrown Twitter-like service, Sina Weibo, has increased.

The government counts on Sina to police the postings and expunge objectionable material that mechanical filtering does not block. Users, however, often find inventive, subtle ways to post views critical of the government.

Nearly all Chinese divers post to Weibo, and a few foreign divers, namely British standout Tom Daley and 14-time U.S. national champion David Boudia, also recently have joined in the proceedings.

“It’s cool to communicate with the Chinese population, and it’s funny to read their posts,” said Boudia, who is also keeping his Twitter account updated with his American phone. “I’m really into social-media networking.”

For the record, swimming governing body FINA has not intervened to open Facebook and Twitter to athletes competing in the worlds.

This article appeared on page B – 7 of the San Francisco Chronicle