Cynthia Bailey's ex-husband, Peter Thomas, has been arrested for DUI, TMZ has confirmed ... after allegedly weaving between lanes and flying through a red light.

According to the docs, obtained by TMZ, officers in Atlanta say they spotted Peter weaving without tags on his SUV last week, so they followed him -- claiming he blew through a red light after exiting the interstate.

PD then pulled him over, and said they detected "a strong odor of an alcoholic beverage" coming from his car ... and said Peter had "watery, glassy, and bloodshot eyes."

After some field sobriety tests, Peter was placed under arrest for DUI -- they say he agreed to a blood test, but when he and cops arrived at the hospital, they say he changed his mind.

They initially brought him to Cobb Adult Detention Center, but he was denied due to blood pressure ... so he was brought back to the hospital and was left with staff -- what a night.

Cops later obtained an arrest warrant for Peter, and he was charged with DUI alcohol, as well as a slew of other traffic-related charges. According to journalist Kiki Monique, Peter was released later on the same day after posting his $3k bail.