I get a lot round here, probably as I'm right by a large park and there are a couple of regulars. I report them to the Dog Wardens.
Try and do it in office hours if possible as ours are difficult and say they won't come out unless you have the dog restrained/captured out of hours and I've spent up to half an hour on hold before now by which point the dog is goodness knows where! I'm sorry but not a chance as you just never know.
Some of them can be quite aggressive. Once caused a fair bit of damage trying to get to next door's dog to the effect they had to replace an entire fence section and get their dog done just in case. I've also had them poo on my front lawn and scuff it up and felt quite threatened - one particular regular was squaring up to a massive bull mastiff (thankfully on lead with owner but if that dog wanted to go then the owner would never have been able to hold it back, it was huge) not long ago.
I call the wardens for a number of reasons, not just the safety of people, especially young children out (especially with the park) and animals - one of my aunt's cats was killed by a dog let run loose - but also the dog itself as who knows what could happen to it on roads or coming into contact with other dogs. For those on the loose I do it so their owner has a chance of tracing them.
There was a nasty incident in the park recently when a loose bull mastiff killed a dog and badly injured a woman trying to protect her young children. You just don't know what even a normally passive animal might do if it feels threatened.