Crews from Connecticut Light & Power and United Illuminating will be working round the clock to restore power to their estimated 760,000 customers without it over the coming days, officials say. At CL&P, some 650,000 customers were without power Monday morning. The number who lost power due to Tropical Storm Irene peaked at 672,000 at 9 p.m. Sunday, CL&P officials say. Meantime, United Illuminating has 110,000 customers without power, and the New Haven electric company doesn’t have yet any estimate on when the power will come back on, said UI spokesman Michael West. Mutual assistance crews arrived Monday morning from Florida, North Carolina, Missouri and Indiana to help UI with the restoration efforts. The company will have an estimate later on Monday when customers can expect their power to be restored, West said. About 75 percent of CL&P crews began their shifts around 7 a.m. Monday to maximize daylight hours and be most productive, they say. The remaining 25 percent will begin their shifts around 3 p.m. and will work through the night, said CL&P President Jeff Butler. In downtown Hartford, the city’s Morgan Street Garage lost power at about 5:30 Monday morning, with a generator providing limited power, officials said. The garage is open to customers but the main Hartford Parking Authority office housed in the building and the payment/hearing office are without power and are closed until further notice.