Three people were injured and hospitalized Friday, early Christmas morning, in a massive explosion in downtown Nashville in the US southern state of Tennessee in what is believed to be an intentional act, according to authorities. 

Police officers were responding to reports of fired shots near 2nd Ave and Commerce Street before 6 a.m. (1230GMT) local time when they saw a suspicious vehicle outside a nearby building.

As the officers alerted the bomb squad, around 30 minutes later a significant explosion happened, Metro Nashville Police Department (MNPD) spokesperson Don Aaron said. 

The police told local media earlier that a parked recreational vehicle (RV) had exploded and caused damage to several buildings. 

Before the explosion, the parked RV was playing a recorded message, "If you can hear this message evacuate now," saying a bomb would detonate within 15 minutes, according to the police. 

"Officers, upon hearing that, decided to evacuate the buildings nearby. So they began knocking on doors, making announcements, having emergency communications with everyone to get people safe," Nashville Police Chief John Drake told the press. 

"Shortly after that, the RV exploded. We had one officer that was knocked to the ground. Another officer is fine," Drake added. 

Nashville Mayor John Cooper said on Twitter MNPD confirmed that the source of the explosion came from a vehicle, adding the force of the explosion knocked an officer to the ground, damaging at least 20 buildings.

"MNPD, FBI & ATF investigating the 6:30 a.m. explosion on 2nd Ave N linked to a vehicle. This appears to have been an intentional act. Law enforcement is closing downtown streets as investigation continues," MNPD said on Twitter.

It added that canine teams are doing protective sweeps in the downtown area where traffic is restricted.

The FBI is taking the lead in the investigation, and it will be working in conjunction with state and local authorities, its spokesman Joel Siskovic said. /aa