As a newspaper headline, it would be something like Youths clash with Belfast police . For normal purposes, I would say The youths clashed with the police in Belfast. Omitting 'the' in either/both cases would not be wrong.
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Jesusengland-I can see no reason why the definite article should be used with youths. We know nothing about these youths and the reference is to no specific group of youths. If we use the before youths, logically it should be used with anyTheyouths clashed with police / the police in Belfast.