Western European leaders failed to reach agreement on proposals to breathe new life into their common political institutions at a summit meeting today that set founding members of the European Community against newcomers such as Britain and Greece.

A communique issued after a two-day meeting here said that a special intergovernmental conference would be held later this year to discuss possible institutional reforms. But European leaders were unable to agree on immediate steps to overhaul the community's cumbersome decision-making procedures.

Although the summit was hailed as a "success" by the host, Italian Prime Minister Bettino Craxi, the community's acting president, British Prime Minister Margaret Thatcher, said an opportunity had been missed to agree on long overdue improvements in the way Europe is run.

"We came here with high hopes," she said. "We believe that Europe can play a much more significant part in the affairs of the world than it is playing now. Progress that we wanted to achieve now has been put off to another conference."

The proposal to call a special conference to change the 1958 Treaty of Rome, which set up the European Common Market, was endorsed by the community's six founding members plus Ireland. Britain, Greece and Denmark, which all joined the community later, were opposed but said they would attend nevertheless.

Western European leaders had hoped that agreement on new decision-making procedures at Milan would help revive the 10-nation community on a new basis, following a crisis that has paralyzed the work of its institutions for much of the past decade. Reforms were regarded as especially necessary in view of the admission of Spain and Portugal at the end of this year.

While expressing disappointment that more was not achieved on the political front, European officials insisted that progress had been made on coordinating technological research and creating a unified market. The summit endorsed a proposal for the dismantling of remaining internal customs barriers by 1992.

As the political leaders met in the splendid medieval surroundings of Milan's Sforzesco Castle, about 100,000 demonstrators paraded around the city in a show of support for the ideal of European unity. The demonstrators, who had assembled in Milan from all over Western Europe, accused the politicians of failing to live up to the ideals of the fathers of the European Community.

"These summits are unable to solve the real problems facing Europe. They talk about political cooperation, when what we need is a European government and a European Parliament with real powers," said Luigi V. Majocchi, secretary general of the European Federalist Movement, which organized the demonstration.

The clear split that emerged at today's meeting between the original signatories of the Treaty of Rome and the newcomers seems likely to revive talk of a "two-speed Europe." The phrase has been used by French President Francois Mitterrand as a way of putting pressure on Britain and other member states perceived as blocking progress toward changes.

In a reference to divisions in the community, European Commission President Jacques Delors said, "Everybody has made it perfectly clear how far they want Europe to go."

Thatcher, objecting to what she saw as attempts to relegate Britain to the slow lane in a "two-speed Europe," said the real dispute in Milan was between governments willing to agree to immediate practical steps to improve the functioning of the community and others holding out for more ambitious goals that might not be attainable. "Women are very practical, as you know," she told a news conference.

Moves to revise decision-making procedures have centered on proposals to introduce majority voting in place of the present system of agreement by consensus. In recent years, ministers from most member nations have resorted to the veto with increasing frequency to defend national or sectional interests.

A proposal to restrict the use of the veto, which originally was introduced at the insistence of the late French president Charles de Gaulle, was blocked by Greek Prime Minister Andreas Papandreou. The proposal now will be considered by foreign ministers.

The European leaders failed to issue any statement on terrorism, despite a discussion about the recent spate of airline hijackings.