Iraq prepared to seal itself off from the outside world and clamp down on movement around the country to ward off threats from insurgents bent on wrecking a referendum on a new constitution. Iraqi detainees and hospital patients began to cast their ballots, two days before the rest of 15.5 million people get their say on the proposed charter for post-Saddam Iraq.Announcing the nationwide lockdown, Interior Minister Bayan Jabr said frontiers would be closed from midnight until Sunday. Businesses were closed for a four-day public holiday and private vehicles will be banned starting midnight tonight. Aside from the armed conflict, a U.S.-brokered deal to defuse Sunni political opposition achieved at least a split in the "No" camp by winning endorsements for the charter from some - in return for a pledge to consider amendments after the vote.