Fidel Castro, defying his own Cuban bodyguards and three other police forces, hugged and shook hands with thousands of people here yesterday and declared: "I feel as if I'm in Havana." He asked Mayor Fournier to call off his Montreal police guards and with them still trailing, he strode across the width of Dorchester St., straddling the central mall, to "meet my people." Still wearing his olive-green army fatigues, and a bronze scapular dangling from his neck, the bearded prime minister of Cuba received rousing welcomes wherever he went. "He could have been killed three dozen times the way he ignored us," a high police official said. As soon as his car, flying the Cuban flag, stopped anywhere, the 32-year-old rebel leader disappeared among the throngs to acknowledge cries of "Viva, Castro" and "Hurray, Fidel."