Pro-Libyan leftist officers are in full control in Upper Volta after ousting their pro-Western president, according to reports reaching here. Capt. Thomas Sankara, who was fired as prime minister in May for his pro-Libyan sympathies, led the violent coup Thursday against President Jean-Baptiste Ouedraogo, who is now under detention. Captain Sankara promised he would lead his impoverished West African country of 6.9 million people on a ''progressive path.'' Sankara has denied that Libya's Col. Muammar Qaddafi engineered the coup, in which 15 people were killed.