Former British prime minister Tony Blair issued a battle cry against a so-called 'hard Brexit' on Friday, calling on voters, businesses and campaigners to "rise up" and back a coordinated effort to temper the terms of, or even halt, Britain's EU exit.In his first major political intervention since Britons voted 52 to 48 percent to leave the European Union last June, Blair said Conservative Prime Minister Theresa May was pursuing "Brexit at any cost," and must be challenged."The people voted without knowledge of the terms of Brexit. As these terms become clear, it is their right to change their mind. Our mission is to persuade them to do so," he said in a speech."This is not the time for retreat, indifference or despair, but the time to rise up in defense of what we believe."May has vowed to start the legal process of leaving the European Union next month, and it is not clear whether the process could then be reversed. Her vision is for a clean break from the bloc, including leaving its single market and customs union.Blair, who won three elections at the head of the Labour Party, has also spoken out in the last 18 months to warn Labour members against electing the hard leftist Jeremy Corbyn as their leader, and to urge voters to shun Brexit. Neither intervention was successful.His speech was aimed at rallying disparate and cowed pro-EU lobby groups into a coherent voice against Brexit, said the organizers of the event, the Open Britain campaign group.He stopped short of calling for a second EU referendum, saying the mechanism for voters to express any change of mind was a "second-order question."