The building, known as CITIC Tower or China Zun, is a 108-storey skyscraper in Beijing's central business district.
It is the headquarters of the state-owned conglomerate CITIC Group.
There was a heavy police presence at the site, with some approach roads closed to cars.
Police prevented some people from taking pictures and asked others to delete those they had taken while ushering people away from the building.
Two glass panels on a high floor were damaged.
There was no immediate official comment and the incident has not been reported on by state media.
A heavy police presence as well as fire engines and ambulances were seen outside the skyscraper, which stands at 528 metres.
A person working in the building told the Associated Press an aircraft crashed into the skyscraper and a fire alarm was triggered.
A courier whom Reuters spoke to near the building said he had rushed over to CITIC Tower about 6pm from a nearby location after hearing a loud crash as an aircraft hit the building.
"It was so loud - louder than fireworks," he said.
He said he had shot a video of the aircraft sticking out of the building but later deleted it because he was scared of getting caught by police.
Another courier whom Reuters spoke to said he had come to the scene after seeing unverified social media images showing wreckage of a small aircraft on a road next to the building.
Social media posts of the building on Friday were quickly removed from Chinese platforms.
On some images, the plane's registration number was visible on the wreckage.
Flight data available online suggested that the aircraft was a Sunward SA 60L Aurora light sport aircraft manufactured in China.
In a social media post, Flightradar24 posted the flight path of the plane, which took off from an airport about 50km east of Beijing.
It headed westward and ended just east of the East Third Ring Road shortly before 6pm.
The flight data provider said the plane crashed into the CITIC Tower just east of a major ring road in a cluster of skyscrapers.
An office worker in a nearby building said she saw a blue tarp over a large object on the road at the side of the building the size of a VW Beetle from her office window about 6.45pm.
"I was on my way down to dinner when someone said a plane had crashed into the next building. So we went to look out the window and saw police cars, ambulances and the blue tarp on the road," the 39-year-old said.
It was unclear if the crash was deliberate or accidental.
Airspace is heavily restricted in downtown Beijing.
One of the bystanders at the scene told Reuters he heard the loud crash too, and that "it's very strange for a plane to fly into this area".
Last month, Beijing authorities enacted new curbs on its already tightly regulated airspace, effectively banning the sale and operation of consumer drones within the capital.
The CITIC Tower is one of the tallest buildings in the world and one of Beijing's landmarks.
It is located next to the headquarters of China's state broadcaster CCTV.
Incidents such as crashes are considered sensitive by Chinese authorities.
Dozens of police cars and several fire trucks lined the roads around the building on Friday.
A police officer later told Reuters journalists to leave.
Asked why, the police officer said: "We all know why!".
with AP and DPA
