Mrs Kadeer denies any involvement in the recent Xinjiang unrest
Uighur activist Rebiya Kadeer has arrived in Tokyo for a visit which has prompted an angry reaction from China.
Mrs Kadeer is expected to use her three-day stay to drum up support for the mainly Muslim minority group.
Beijing says the 62-year-old was behind a recent outbreak of deadly ethnic unrest in Xinjiang province.
But Mrs Kadeer, once a businesswoman in China and now leader of the exile group the World Uighur Congress, has denied any involvement.
Wearing a traditional Uighur hat, she was greeted at the airport by a small group of supporters carrying Uighur flags.
"I came here to let the Japanese people know the terrible conditions that the Uighurs are suffering," Mrs Kadeer was quoted as saying by Japan's public broadcaster NHK.
She said she wanted to "let the people know how many Uighurs are actually killed and arrested", referring to recent unrest in Xinjiang, western China.
Mrs Kadeer's visit to Japan will include a news conference and meeting with members of the ruling Liberal Democratic Party.
But one supporter told the AFP news agency earlier the itinerary could change according to events.
"We are a bit worried about ensuring her safety as she has been attacked by mobs in the past elsewhere," said the unnamed supporter.
The BBC's Roland Buerk in Tokyo says that ties between Japan and China have improved in recent years, despite tensions over wartime history and territorial disputes.
But Mrs Kadeer's visit threatens to strain relations between the major trading partners, says our correspondent.
Japan's Foreign Ministry Press Secretary, Kazuo Kodama, said Mrs Kadeer had been granted a visa "based on the usual procedure", so the visit should not cause any diplomatic problems.
The Uighurs say their culture is threatened by Han Chinese migration
Mrs Kadeer, who now lives in the US, was imprisoned in China for six years until 2005 on charges of endangering national security.
She was nominated for the Nobel Peace Prize in 2006 and many Uighurs see her as the international figurehead their movement previously lacked, says the BBC's Jill McGivering.
But Beijing views her as a troublemaker and has accused her of organising the violent unrest in Xinjiang.
On Monday, Chinese ambassador Cui Tiankai called her a criminal.
"How would the people of Japan feel if a violent crime occurs in Japan and its mastermind is invited by a third country?" Mr Cui was quoted as saying by Kyodo news agency.
He hinted that the visit could harm relations between China and Japan.
"We must prevent important matters that should be worked on together from being disturbed by a criminal or attention to our common interests from being diverted," he said.
Nearly 200 people - mostly Han Chinese - died in the clashes with Uighurs in Xinjiang, according to Chinese officials. Uighur exiles say hundreds of their people were killed.
The unrest began on 5 July during a protest by Uighurs over a brawl in southern China in late June in which two people were killed.
China's Uighurs are concentrated in Xinjiang but complain their rights and culture are being overridden by an influx of Han migrants from outside the region.