The accident happened at 8:45 p.m. local time when one of the trains, which was on a test drive and had only one driver, collided with a train carrying passengers.
The two trains, which are operated by Prasarana Malaysia Berhad, were traveling at 40 km (about 25 miles) per hour when they collided in a tunnel between Kampung Baru and KLCC stations, state media said.
Some 166 people were slightly injured and were sent to nearby hospitals for treatment, including Kuala Lumpur Hospital. Forty-seven passengers were seriously injured, state media added.
Videos and photos posted on social media showed several passengers were bleeding and broken glass panels inside the trains.
Malaysian Prime Minister Tan Sri Muhyiddin Yassin called for a full investigation to find out the cause of the crash.
“I take this accident seriously and have asked the Ministry of Transport and Prasarana Malaysia Berhad to conduct a thorough investigation,” he said, Bernama reported.
Transport Minister Wee Ka Siong said the collision was the first major accident in 23 years of operating the metro system, and he promised a full investigation, Reuters reported.