China has arrested dozens of dissidents and campaigners ahead of a visit by United States President Barack Obama, who is under pressure in the US to challenge China on its human rights record.

China has arrested dozens of dissidents and campaigners ahead of a visit by US President Barack Obama, who is under pressure in the US to challenge China on its human rights record.

Among those detained is the head of an activist group for parents whose children fell ill after drinking contaminated milk. According to his wife, Zhao Lianhai has been detained with any charge.

Six children died and 300,000 fell ill after taking milk tainted with the chemical melamine. The scandal emerged in September 2008. A dissident who wanted to organise a human rights rally in a park in the capital Beijing has also been arrested.

Crates of contaminated powder milk (Photo: ANP)