Those are aphids. Probably melon aphids, Aphis gossypii. A pretty bad infestation too. The first thing I would do is to brush as many as you can from the plants. Brush them onto a sheet of paper and squish them or if you can't stand that then brush them into a bowl of soapy water. If you just brush them off then they will climb back onto the plant. You can use a nice stiff paintbrush to do this, or just use your fingers. Next I would spray the plants with an insecticidal soap to try and kill off any remaining insects. Repeat the brushing and spraying as necessary. Aphids are prolific and if even just a few are missed on the plants they will soon multiply so be persistent, its probably going to be an ongoing battle this summer.

Just a note, ladybird beetles love aphids and if you can get hold a native species in bulk you could release them. You could even try collecting them yourself but under no circumstances should you import ladybirds from another country. I doubt this would be possible anyway but it is suggested in another of the answers and its not a good idea. Introduced species can wreak havoc and severely damage native insect populations - an example (albeit an extreme one) is the Harlequin ladybird which was introduced to the US from Asia as a biological control agent and has since spread to countries all over the world - including the UK. This ladybird is so effective at eating aphids that it outcompetes native ladybirds and reduces their populations.