HARARE, Zimbabwe — Giddy with joy and finally free to speak out, vast throngs of demonstrators turned Zimbabwe’s capital into a carnival ground on Saturday in a peaceful outpouring of disdain for President Robert Mugabe and calls for him to quit immediately. Still clinging to his now-powerless post, the longtime leader has been put under house arrest by the military. People in Harare, meanwhile, clambered onto tanks and other military vehicles moving slowly through the crowds; danced around soldiers walking in city streets, and surged in the thousands toward the building where Mugabe held official functions, a symbol of the rule of the now-93-year-old man who took power after independence from white minority rule in 1980.