Google Inc. faces intensifying challenges in China after a popular website and Internet browser company there replaced the search service for its own technology.

Qihoo 360 Technology Co. launched its own search engine last week and is in the process of making it the default on its website and browser, said Chief Financial Officer Alex Xu in a Tuesday interview. The Beijing company, whose Internet browser reached about 270 million monthly users in the first quarter, previously offered Google's search technology as the first...