Twitter co-founder Jack Dorsey arrived on a visit to China on Thursday and complained of the blocking of his popular service in an online exchange with Chinese artist Ai Weiwei.
"Hello, Shanghai. Twitter is blocked here in China so I can't read any tweets," Dorsey wrote on his Twitter feed @jack, which has 1.8 million followers.
"Welcome to land of no twitter!" Ai responded on @aiww, where he has over 119,000 followers.
"Yes, it's unfortunate and disappointing," said Dorsey, the executive chairman of San Francisco-based Twitter.
"We really miss you here," Ai said. "Have a nice rest, (and) enjoy shanghai...a city without culture, but a lot of money.
"Let's make sure China has access to Twitter sooner than North Korea," Ai said in a final tweet to the Twitter co-founder.
Ai's activism has made him a thorn in the side of the Chinese authorities and he disappeared into custody for 81 days last year as police rounded up dissidents and lawyers amid online calls for Arab-style protests in China.Upon his release in June, the artist was charged with tax evasion. His case is currently being reviewed by the Beijing tax bureau.
Twitter and Facebook are among the social networking services blocked in China, which has half a billion Internet users, the world's largest online population.