"We are now closing down the Broad Street entertainment zone, and we are asking people to go home from that area," a West Midlands police spokeswoman said on Saturday.
There were about 200 bars and clubs in the zone, which spans about half a square mile, she said.
"They are all being evacuated," she said. "In the wider Birmingham city centre people are being encouraged to leave and go home."
Police had issued a statement at 8.00pm (1900 GMT) warning they had received intelligence to suggest a threat to the city centre.
At that time, they closed off the inner ringroad, stopping any traffic entering the city. Any vehicles within the zone would have to pass a police cordon, she said.
"No vehicles are going into the inner ring road," the police spokeswoman said.
Sky News reported roadblocks were in place and helicopters were flying over the area.