The Shenzhou 7 spacecraft, China's third manned mission, blasted off atop a Long March 2F rocket shortly after 9:00 a.m. EDT under clear night skies in northwestern China.
The spacewalk by one of the astronauts is expected to take place either on Friday or Saturday.
Underscoring the mission's heavy political overtones, Chinese President and Communist Party head Hu Jintao was shown live on state television hailing the astronauts at the launch site near the northwestern town of Jiuquan.
"You will definitely accomplish this glorious and sacred mission. The motherland and the people are looking forward to your triumphant return," Hu told the three, who were dressed in their flight suits and behind glass to avoid germs.
The mission, expected to last three to four days, is devoted almost entirely to carry out the 40-minute spacewalk.
The spacewalk is expected to help China master the technology for docking two orbiters to create China's first orbiting space station in the next few years.
The spacewalk could happen either Friday or Saturday depending on how well the astronauts adapt to weightlessness and other physical demands of their environment, according to the China Manned Space Engineering Office.
The astronauts would return to Earth soon after the spacewalk, the office said.
The two astronauts who don spacesuits for the Shengzhou 7 spacewalk will be supported by Russian experts throughout the mission. Only one will actually leave the orbiter module to retrieve scientific experiments placed outside.
One of the astronauts will wear China's homemade Feitian suit, while the other will wear a Russian-made suit.
Fighter pilot Zhai Zhigang, an unsuccessful candidate for the previous two manned missions, has been touted by the official Xinhua News Agency as the leading astronaut to carry out the spacewalk.
Zhai and fellow astronauts and fighter pilots Jing Haipeng and Liu Boming - all age 42 - were introduced to journalists at news conference late Wednesday.
A decade of training together ensured effective, smooth cooperation between the three, Liu said.
"The Shenzhou 7 mission marks a historic breakthrough in China's manned space program," Zhai said. "It is a great honor for all three of us to fly the mission, and we are fully prepared for the challenge."