Sweden has condemned China's "brutal" and "arbitrary" detention of a Swedish citizen who sold politically sensitive books and whose seizure while he traveled with diplomats has stunned Western governments.

The sharply worded statement from Foreign Minister Margot Wallstrom questions the rule of law in China and marks a tougher stance from Sweden two weeks after Chinese security agents seized Gui Minhai as he traveled to Beijing with two Swedish diplomats.

Wallstrom characterized the detention on Monday as "contrary to basic international rules on consular support" and demanded that China disclose Gui's whereabouts and his alleged crime.

Fifty-three-year-old Gui sold gossipy books about Chinese leaders from Hong Kong before he disappeared the first time in 2015, when he was believed to be abducted by Chinese agents in Thailand.