The action, called partly in protest at European Community farm policies, was a humiliation for the government in its campaign to win approval for the Maastricht treaty of European union in a referendum on 20 September.
Farmers protesting against falling fruit prices prevented delegates from reaching the congress centre in this southern city. They snatched and burnt some conference documents after riot police injured the local farmers' leader in a dawn clash.
Jean-Pierre Boisson, his nose bandaged and his shirt stained with blood, told reporters the farmers would not budge until the Prime Minister, Pierre Beregovoy, met demands for emergency aid, debt relief and a ban on imports from outside the EC.
'It is a provocation to hold this meeting here. We demand that it be cancelled,' said the farmers' association secretary-general, Francis Durand.
The Industry Minister, Dominique Strauss-Kahn, met a deputation led by Mr Boisson to discuss the farmers' grievances and voiced understanding for their problems.
'We have perhaps found a solution, I hope so,' said the minister. 'It is normal for the government to be concerned by the problems of the farmers . . . but it is also normal that the government should ensure by all necessary means the freedom to hold meetings.'
Riot police sealed off the square outside the Papal Palace, and the Socialist Party leader, Laurent Fabius, remained barricaded inside the heavily guarded government prefect's office.
Socialist Party officials said they would try to start the summer school later, either at the congress centre or elsewhere in Avignon.
A host of Socialist leaders, including Jacques Delors, the President of the European Commission, and several cabinet ministers are due to address the three-day summer school.
French farmers have staged sporadic protests since May, when the EC agreed on sweeping farm reforms, including subsidy cuts.
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