The comedian managed to present most of the two-and-a-half-hour show before he finally dropped hints that it was an April Fool's prank.
Listeners reportedly took to the Radio 2 message boards, commenting on Bruce's erratic behaviour, and questioning whether the usual presenter was drunk.
Brydon had still not revealed his true identity by the end of the programme, where he handed over to the next presenter Jeremy Vine, who he called Tim - the name of Vine's comedian brother.
Brydon also conducted a spoof interview with 'Terry Wogan', who was actually played by Peter Serafinowicz.
The real Bruce later joked: "It's been wonderful to be able to listen to the radio - tuning into someone entertaining in the morning for a change. I'm offering him a contract to share the honours on a regular basis."
Bruce will join Chris Evans for Radio 2's coverage of the royal wedding on April 29.
Brydon's panel comedy Would I Lie To You? was recently renewed for a fifth series.