Hundreds of demonstrators took to the streets of Istanbul on Thursday to protest China's treatment of mainly Muslim Uighurs in Xinjiang, an AFP journalist said.

Around 500 people gathered at the city's Beyazit square, holding up pictures of their missing families and unfurled banners reading: "Where's my family?", "Free my family" and "Shut down concentration camps!"

The group, which included children, called for an end to the crackdown in China's northwestern region -- where more than one million Uighurs and other mostly Muslim Turkic-speaking residents are believed to be held in camps.

China is running hundreds of detention centres in northwest Xinjiang across a network that is much bigger than previously thought, according to research presented last month by the Australian Strategic Policy Institute (ASPI) thinktank.

The number of facilities is around 40 percent greater than previous estimates, the research said, and has been growing despite China's claims that many Uighurs have been released. Beijing has denied the existence of detention sites. The government says they are vocational training centres used to counter extremism.

Not many Muslim leaders have openly criticised the treatment of Uighurs with the exception of President Recep Tayyip Erdogan of Turkey, which has linguistic and cultural connections with the Uighurs.

Mukerrem Kutar, one of the protesters holding up pictures of her missing relatives, said: "I cannot get news from my family, I have no news at all from my brother, his son and their whole family." She told AFP: "I have no idea if they are alive, dead, in camps. I want to find out where they are."

Yunus Abduzahir, 25 and a student, said he had lost contact with his family since 2006. "The last time I heard from them, my mother, my father and my older brother were detained and my brother was sent to a forced labour camp," he said.

"My mother, had to leave her two-year-old kid and had to live in a different city, I don't know where she is...  She had a two-year-old she had to look after, but I imagine that in spite of that they took her and sent her to a concentration camp."