European Community leaders gathered today for a summit aimed at launching the trading bloc on the road to a federated Europe. Britain, the prime objector, is in a more conciliatory mood since the resignation of Margaret Thatcher. The 12 leaders beginning their two-day meeting today in the flag-bedecked Italian Parliament also will consider granting a $2.4 billion aid package to the Soviet Union and increased help to other East European countries. The Europeans received a new exhortation from President Bush to stand firm on the Persian Gulf crisis. In a letter Thursday to Italian Premier Giulio Andreotti, current holder of the rotating summit chairmanship, Bush said Iraq's blanket release of foreign hostages should not lessen pressure on Iraq to pull out of Kuwait. ``We must continue to maintain a very rigid attitude,'' Bush said, according to Italian officials. British Prime Minister John Major, who succeeded Mrs. Thatcher Nov. 28 and is virtually unknown to the other summiteers, joined them at the opera for a performance of ``Tosca'' on Thursday night. Major, 47, was holding separate prior talks with Andreotti, Dutch Prime Minister Ruud Lubbers and German Chancellor Helmut Kohl. Kohl and French President Francois Mitterrand are pressing for economic union and a single currency, perhaps by the year 2000, and for a form of political union that would strip member nations of many sovereign powers. Major, keen to avoid a repeat of the Britain-versus-the-rest showdown that helped topple Mrs. Thatcher after the summit in Rome in October, was optimistic despite Britain's continued opposition to key unity proposals, his aides said. ``We see ways in which significant progress can be made,'' a senior British source said Thursday night. ``But there are areas where we will reaffirm our positions quite clearly, including our opposition to having a single currency imposed upon us,'' he said, speaking on condition of anonymity. The European Parliament in Strasbourg, France, on Thursday urged the 12 EC heads of state to work on restarting talks to liberalize world trade. The global trade negotiations broke down in Brussels last week, in part due to the EC's refusal to cut farm subsidies enough to satisfy the United States. The talks are to resume at a lower level next month in Geneva. Soviet aid, which topped the agenda of this summit, was given fresh impetus by the U.S. decision Wednesday to give the Soviet Union $1 billion in loan guarantees to buy American farm products and manufactured goods. Andreotti's spokesman, Pio Mastrobuoni, said he was confident of agreement, despite some arguments that the Soviet Union needs help in managing its huge resources, including distributing its harvest, rather than handouts and loans it will have trouble repaying. On financial union, Andreotti cornered Mrs. Thatcher at the October summit by getting the others to agree to establish a single central banking system - the prelude to one currency - by January 1994. Mrs. Thatcher objected loudly. In a change of tone, Major planned to argue the British alternative of a European currency that would circulate alongside the 12 existing currencies, without ruling out an eventual sole currency. On political union, Britain has shifted toward French and German proposals for giving the EC a formal role in security. And aides said Major was confident Britain will not be alone in opposing other proposals that would give the community far-reaching powers over issues ranging from health, social policy and education to taxation and transport. The EC nations are Italy, France, Britain, Ireland, Spain, Portugal, Belgium, Luxembourg, the Netherlands, Greece, Denmark and Germany.