The explosion rocked the upscale Las Condes neighbourhood of Santiago at 8:50 am (1150 GMT) close to a church.
Police said there were few people present and no injuries or deaths were reported, though they were investigating who may have planted the bomb.
"This incident should be investigated and punished. Nothing justifies such a violent act," prosecutor Raul Guzman said.
The bombing is the latest in a spate of explosions that have left Chile on edge, in the worst violence to hit the nation in a quarter century.
The subway attack was the most destructive of some 200 unsolved bombings that have targeted banks, gyms, embassies and restaurants in the South American country over the past five years.