A leading British politician urged Western governments today to try financial pressure to force authorities in Poland to ease their military crackdown and begin negotiations with church and Solidarity leaders.

Shirley Williams, a former Labor government Cabinet minister who is a leader of Britain's new Social Democratic Party, told more than 10,000 demonstrators at a "Solidarity with Solidarity" rally here that U.S. and European banks should be told not to renegotiate any more than the estimated $27 billion in debts owed by Poland until the military government meets Western demands.

"The banks should not lend another penny to Poland or any other Eastern European country," Williams said, until the Polish leader, Gen. Wojciech Jaruzelski, opens negotiations and frees those who have been arrested, including Solidarity leader Lech Walesa, who reportedly is under house arrest.

A group of U.S. officials led by Undersecretary of State Lawrence Eagleburger is due to arrive here Monday to begin consultations with the NATO allies on whether they should take collective economic and political actions such as those suggested by Williams.

The response of the European allies to military rule in Poland so far has been limited to statements of strong concern and demands that those detained be freed and negotiations begun.

The protest rally and march to the Polish Embassy here in bitterly cold weather was one of the largest of many such demonstrations in European cities this past week. All of Britain's major political parties were represented, along with trade unions, left-wing political groups and Polish emigres.