World Bulletin / News Desk
Access to and from the visitor center and all units of the building were closed at 4.15p.m. (2115GMT).
No details about the incident were released by authorities but some media sources reported that police told lawmakers the lockdown was triggered by an incident close to the complex.
ABC News reported that police said they issued lockdown orders after receiving a call from a man reportedly with a machine gun.
The House of Representatives was in the middle voting and Senate lawmakers were delivering speeches on the floor of its chambers.
Last Friday, the Capitol was temporarily locked down after police believed a woman took a weapon into the complex.