You'll likely need an angle grinder with a metal cutoff blade (bolt cutters can't handle a good lock, though if it's on a chain, the chain might be a weak link).

The article said there's security on the premises, so be prepared for that sort of trouble (make sure you've got an emergency number and people who know what's going on in case you're arrested). Though, I have a hard time seeing how restoring legal public access could be considered a crime. Maybe just bringing witnesses and cameras will be enough to protect you from security.