British astronaut Tim Peake has run a marathon in space in record time, strapped to a treadmill on the International Space Station as thousands ran the London Marathon below.

Peake opened the race on Sunday by counting down in a video message as runners waited at the start line in the British capital.

He then joined them 400 kilometers above Earth on a simultaneous feat aboard the space station, wearing weights to counter the zero gravity conditions.

"Hello #London! Fancy a run?" Peake wrote on Twitter before the race, accompanied by a photograph of London from above.

He followed up with a message sent after he completed the marathon, in which he noted that while he had run the 42 kilometers, the space station had traveled almost 100,000 km.

"Congratulations to everyone in #LondonMarathon & #teamastronaut," he wrote. "Gonna sleep well tonight!"

Peake is the second person to complete a marathon in space, after US astronaut Sunita Williams ran the Boston Marathon on the International Space Station in 2007 in 4 hr, 23 min, 10 sec.

But Peake ran a faster marathon in space in 3 hr, 35 min, 21 sec, according to estimated times posted on the website of the European Space Agency.

Guinness World Records wrote on its website, "ESA astronaut Tim Peake has achieved a brand new Guinness World Records title for the fastest marathon in orbit."