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.
Beijing's municipal government did not immediately respond to a faxed request for comment from Reuters outside of business hours.
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.
The person spoke on condition of anonymity out of fear of retaliation.
A courier whom Reuters spoke to near the building said he had rushed over to CITIC Tower about 6pm local time from a nearby location after hearing a loud crash as a aircraft about the size of a car 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 social media.
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.
A search of the building's name on the Xiaohongshu app returned only posts dated Thursday.
The CITIC Tower is one of the tallest buildings in the world and one of the landmarks of the Chinese capital.
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 told Reuters journalists to depart from the scene.
Asked why they had to leave, the police officer said: "We all know why!".
with AP and DPA
