RIO DE JANEIRO, Aug 26 (Reuters) - Brazil President Jair Bolsonaro on Thursday said Brazilians need not worry, as the Sept. 7 nationwide marches by his supporters will not be violent. Concerns have been growing that Bolsonaro"s supporters, riled by frustrations with the Supreme Court, federal election authorities and lawmakers, could strike out during the marches. Tensions are running high in Brazil, which has been hit hard by the coronavirus pandemic and years of graft and economic underachievement.