If you get arrested for singing on a protest, that's all to the good. It might even get the rest of the orderly crowd singing.

On CND marches I always used to sneak up behind people shouting out the kind of slogans I didn't like the sound of and start singing. More often than not it worked.

Never got arrested, but then the laws weren't so repressive in those days.

On the other hand the police these days are probably more worried about their public image and about looking like total prats, so I doubt if you've got anything to worry about.