Flacco, who hit 28 of 46 passes, threw for a score and ran for another.
A 63-yard interception return for a touchdown by Cary Williams with 15 seconds left in the third quarter turned a slim 16-10 Baltimore lead into a 13-point cushion.
The pass by rookie Brandon Weeden was intended for Travis Benjamin, but Williams jumped the out pattern and raced down the right sideline with his first career interception.
After a scoreless first quarter, Baltimore went up 6-0 on an 18-yard touchdown pass from Flacco to Torrey Smith. The conversion failed.
On the Ravens’ next drive, rookie Justin Tucker kicked a 45-yard field goal.
Cleveland got back into the game with an 11-play, 94-yard march directed by Weeden, who connected with Greg Little for 43 yards before rookie Trent Richardson ran in from the 1 to cut Baltimore’s lead to 9-7.
On the opening drive of the third quarter, Flacco took the Ravens 89 yards in 13 plays for a 16-7 lead. Flacco completed five passes during the march, including three to Anquan Boldin for 60 yards, before the quarterback ran in from the 1 on a third-and-goal.
The only scoring in the fourth quarter came from Cleveland kicker Phil Dawson, who booted field goals of 50 and 52 yards. He also kicked a 51-yarder in the third quarter, accounting for all of Cleveland’s points.
The Browns were forced to play most of the game without wide receiver Joshua Cribbs, who left with a head injury after absorbing a hard hit late in the first quarter.
Cribbs was running back a punt when he lost his helmet while taking a shot from the shoulder of Dannell Ellerbe. Cribbs lay prone for several minutes before being helped from the field.
Cribbs lost the ball on the play, and Jameel McClain recovered the fumble, but Flacco gave it back by throwing an interception.