Washington: Space Shuttle Discovery astronauts have begun their mission's first spacewalk, in which they will install a new component on the International Space Station (ISS).

American Robert Curbeam and Christer Fuglesang, the first Swede in space, began the spacewalk at 2031 GMT Tuesday and were expected to wrap up the six-hour event around 0241 GMT Wednesday.

Inside the station, astronaut Joan Higginbotham will use a robotic arm to guide a two-tonne truss into place. Curbeam and Fuglesang will help guide the truss and will bolt it into place and complete its installation.

The truss they install will allow astronauts on a future mission to move another truss and its solar arrays to another location on the station.

Discovery lifted off late Saturday from the Kennedy Space Centre at Cape Canaveral, Florida, for the construction mission to the ISS.

The flight's primary mission is to hook up the ISS' permanent electricity-generating system, replacing a temporary power system operating since the space station went into orbit in 1998. Three spacewalks are planned, including two to rewire both halves of the station.

Other spacewalks are scheduled for Thursday and Saturday.

NASA has described Discovery's rewiring mission at the ISS as one of the most complex and difficult in the history of space flight.

Discovery is also to deliver Indian origin astronaut Sunita Williams to replace German astronaut Thomas Reiter after five and a half months in residence on the space station, which has a rotating crew of three astronauts.

Discovery's flight is the second working shuttle mission to the ISS since NASA returned to flight in summer 2005 after the shuttle fleet was grounded for two years following the Columbia disaster, which killed all seven crew members.

NASA spent most of the last year testing new safety systems. A September flight by the shuttle Atlantis marked the resumption of ISS construction.