They marched through some areas in Yenagoa, the state capital.
The party loyalists carried banners and chanted victory songs.
David Lyon, the APC candidate, won the November 16, 2019 governorship election.
But the Supreme Court declared Diri and the People’s Democratic Party (PDP) winner after ruling that the APC Deputy Governor-elect, Biobarakuma Degi-Eremienyo, presented a forged certificate.
On Monday, the State Election Petition Tribunal ordered the Independent National Electoral Commission (INEC) to conduct a fresh poll.
It declared that INEC wrongly excluded the Advanced Nigeria Democratic Party (ANDP) candidate.
Meanwhile, the governor has directed his lawyers to appeal the judgement.
Diri stated that he has implicit confidence in the judiciary and would triumph.