(S (S (S (BASENP (NN Riot) (NN police)) (VP (VP (VBD dispersed) (BASENP (DT the) (NN crowd))) (PP (IN with) (BASENP (NN tear) (NN gas))))) (SBAR (IN as) (S (BASENP (NNS clashes)) (VP (VBD began))))) (PERIOD .))