Is time enough to visit the Beijing Tiananmen Square and Forbidden City within 6 hours?

I will have 8hours lay over in Beijing, I want to go out of the airport for few hours sightseeing. I will arrive beijing 12:00 noon, and leaving 20:00. with that time (including checking out and checking in at the airport) would that be enough time to see TIAN'ANMEN SQRE and FORBIDDEN CITY?, then go back to the airport by 6:00pm?
Thank you.