The building was evacuated and Stone was arrested after he walked into the Stormont parliament building with a bag and left it on the floor, the BBC reported Friday. Police said there were reports of an unspecified device inside the bag.
Stone, a former Ulster Freedom Fighter prisoner who supports British rule in Northern Ireland, reportedly shouted "no surrender" and was seen carrying a firearm, but it was not known Friday whether the weapon was real or a toy, Sky News reported.
"It seems that Michael Stone has gone on the rampage again, in a very dangerous way. But he was stopped," Irish Taoiseach Bertie Ahern said.
"It just shows you exactly what we are trying to get away from in Northern Ireland."