The Broncos, having already clinched the AFC West title, won their eighth game in a row and improved to 10-3.
Manning directed the Broncos 68 yards in 10 plays on the opening possession of the contest and finished off the march with his only scoring pass of the night -- a 6-yarder to Joel Dreessen.
Prater kicked a field goal in the first period, another in the second and two more in the third -- the longest being 43 yards. After taking a 13-7 halftime lead, Denver put the game away with a 13-point third period that included a 1-yard scoring run by Knowshon Moreno.
Moreno, playing in just his fifth game of the season, ran for 119 yards on 32 carries
Manning, nearing the end of his first season with the Broncos and enjoying a potential MVP campaign, completed 26-of-36 passes for 310 yards with an interception.
He began the year tied with Dan Marino for the most 300-yard passing games in NFL history at 63 and has gone on to shatter the record.
During Thursday's game Manning also became the fastest player in league history to reach 5,000 completions. He did so in his 221st game, 18 fewer than it took Brett Favre to get to that total.
Oakland slipped to 3-10 with its sixth consecutive loss. The Raiders have the second-worst record in the NFL.
Raiders quarterback Carson Palmer completed 19-of-30 passes for 273 yards with two touchdowns and an interception.