Officers in climbing gear scaled the roof of the five-storey former police station at Beak Street, while others armed with a chainsaw tried to bring the door down.
Police had been issued a search warrant on the grounds that anti-capitalist protesters squatting inside may have had weapons.
One protestor tried to jump off the roof and had to be wrestled to safety by officers.
Powers to stop and search in anticipation of violence and to require the removal of disguises were authorised on Tuesday morning and remain in force.
Demonstrators opposed to the G8 are taking action ahead of next week's two-day summit in Northern Ireland.
Rights to peaceful protest do not extend to violent or threatening behaviour and the police have powers to deal with any such acts.
A group calling itself StopG8 has planned a "Carnival Against Capitalism" in central London.
Businesses and banks have warned staff to take precautions after similar protests in recent years led to violent clashes with police.
Officers positioned themselves outside Fortnum & Mason to block protestors from attacking the shop, leading to a tense stand-off - more protestors gathered outside the BP headquarters in St James's Square.
According to the Met Police, nearly 1,200 officers were on duty and a total of 32 arrests have been made over violent disorder, possession of bladed articles and a variety of public order offences.
A Home Office spokeswoman said: "Peaceful protest is a vital part of a democratic society, providing it is conducted within the law.
"But protesters' rights need to be balanced with the rights of others to go about their business without fear of intimidation or serious disruption to the community."
Security has already been stepped up in and around the Lough Erne Resort near Enniskillen, where the leaders of the eight of the world's most powerful countries will hold talks.
Belfast has also prepared for thousands of protestors to descend on the city, with round-the-clock protection for key landmarks and businesses.