This info is for the United States ONLY: There are NO federal laws in the US regarding the ownership or carrying of any class (any power) of laser. However, there IS a law against selling (but not buying or owning) a Laser of greater than 5mW that is marketed as a "pointer".
Also, you will want to check your state statutes to see if there's any kind of limitation imposed by your state. Additionally, check county & city ordinances for any local limitations.
There are federal, state, and local laws covering illegal use of a laser, many simply state "Laser" & do not give a maximum wattage in any of the statutes I've looked at.
For example: on February 14, 2012, President Obama signed Public Law 112-95, the FAA Modernization and Reform Act of 2012. Section 311 amended Title 18 of the United States Code (18 U.S.C.), Chapter 2, § 39, by adding § 39A, which makes it a Federal crime to aim a laser pointer at an aircraft. The crime is punishable by a fine up to $250,000 and five years imprisonment. That law covers any laser of any output level. Anything that happens because of pointing a laser at an aircraft (crash, vision damage, etc...) is punishable separately.
Florida state laws regarding improper use of lasers are also for any laser of any output level.
In fact the only place I'm able to find any limitation on laser power in usage, is in the use of lasers for public display's &/or laser light shows which are limited to 5mW or less.
Basically, if you wanted to own a 500W laser, you likely can. If you want to use said laser in a safe manner that has no potential to harm others, you likely can. But as soon as you point the beam at another person, be they sleeping, standing, sitting, riding in a craft of any kind, or driving / piloting a craft of any kind... 99% chance you're breaking at least one law & probably more.
Again, this is only for the United States.