Is it legal to be in only underwear in public?

In most places in the western world, yes it is legal to be in underwear in public. Or to be more precise, it is not illegal, there being a subtle difference. But that needs some clarification. Doing so isn't usually classed as indecency. That is presuming of-course that the underwear isn't see-through, tight or so minute that private parts are clear or exposed.

Having said that just about everywhere has some law that can be used to stop you in your tracks - if the police are so minded. The kind of law would be causing a public nuisance or alarm - called different things in different places. Such laws are often open to interpretation.

In the US these days there are a great many underwear events - from fashion stunts, charity runs and walks to the annual No Pants metro ride - not to mention NY's very own Naked Cowboy who struts around wearing nothing but a pair of tighty whities and a guitar in all weathers. It is doubtful if many of them have been arrested for indecency - or even if the participants themselves consider what they are doing is indecent.

The use of nuisance laws would hardly likely to be used against those taking part in a mass event especially one for charity. Basically it would come down to context. If you are up to no good and out to cause alarm then maybe what you are doing is illegal. If it is simply harmless fun and especially in a group the police might have a hard time securing a prosecution - and indeed made to look very foolish if they tried.