China has detained several dissidents and campaigners ahead of US President Barack Obama's much-anticipated first visit to the country, their relatives and close contacts told AFP Saturday.

Obama arrives in Shanghai on Sunday and moves onto Beijing the next day for a four-day maiden presidential trip during which he has been urged to raise human rights with the Asian giant's top leadership.

But as the visit drew close, the head of an activist group for parents whose children were sickened by tainted milk in China had been detained, his wife told AFP.