Under the Online Safety Act, the regulator can now take action over illegal content and that includes videos promoting terrorism or banned extremist groups.
All websites will now have to show they have systems in place to remove illegal material. If they fail to do so, the regulator can get court orders to block platforms or impose fines of up to £18m.
And from the summer all sites must have robust age verification systems to prevent children accessing a range of content.
But critics believe the legislation itself is weak and that Ofcom is not being robust enough in how it plans to police sites.