Rebiya Kadeer, the exiled leader of China's Uighur minority, has arrived in Japan on a visit that has sparked anger in Beijing.
China accuses US-based Kadeer of orchestrating the deadly clashes between Uighurs and Han Chinese in Urumqi in the northwest region of Xinjiang earlier this month.
She denies the allegation, and cites reports of far higher death tolls in China's worst ethnic violence in decades.
Kadeer, 62, who heads the World Uighur Congress, is expected to call for support for China's largest ethnic minority during her five days in Japan.
She is scheduled to address Japanese journalists in Tokyo on Wednesday and give a lecture Thursday, her supporters in Tokyo told the AFP earlier, adding that there were concerns for her safety.
China routinely criticises international trips by the Dalai Lama, the exiled Tibetan spiritual leader, whom it blames for instigating deadly ethnic violence in Tibet last year.
"How would the people of Japan feel if a violent crime occurs in Japan and its mastermind is invited by a third country?"
Cui Tiankai, China's ambassador to Japan
It has rarely commented on Kadeer's travels before.
Japanese officials however do not generally meet with the Dalai Lama and a foreign ministry spokeswoman said there were no plans for official meetings with Kadeer, who spent six years in a Chinese jail before being freed in 2005 under pressure from the United States.
But any contact between the Uighur leader and Japan's ruling party members is bound to affect diplomatic ties between the major trading partners.
On Monday China's foreign ministry expressed "strong dissatisfaction" with Japan's decision to allow Kadeer's visit despite Beijing's "repeated and serious representations".
"Ignoring China's repeated and solemn representations, the Japanese government persisted in allowing Rebiya [Kadeer] to engage in anti-China separatist activities," Qin Gang, a foreign ministry spokesman, said in a statement.
Qin's statement followed similar criticism by China's ambassador to Japan, Cui Tiankai, in an interview with Japanese media earlier on Monday.
|Close to 200 people died in China's worst ethnic violence in decades [EPA]
"How would the people of Japan feel if a violent crime occurs in Japan and its mastermind is invited by a third country?" the Kyodo News agency quoted Cui as saying.
"The matter can be considered easily when you think from the other person's viewpoint ... she is a criminal."
Japan's government has said the deaths caused by days of bloody unrest in Xinjiang were "very regrettable" and called for a peaceful resolution of the situation, urging China to protect the human rights of the mainly Muslim Uighurs.
Japan and China, major trading partners, have in recent years worked to improve ties often strained by rows over historical and territorial disputes.
An Australian film festival has also come under fire for a planned appearance by Kadeer next month related to a documentary about her titled the "10 Conditions of Love".
Over the weekend, hackers posted a Chinese flag and left anti-Kadeer slogans on the website of the Melbourne International Film Festival and organisers said the site remained partially disabled on Monday.
Four Chinese directors also withdrew their films in protest amid an escalation of protests against the documentary which premiered on Sunday.