BRUSSELS (Reuters) - The European Union's foreign policy chief said on Tuesday Egypt's sentencing of 683 people to death breached international law and urged Cairo authorities to ensure defendants' rights to a fair and timely trial.

An Egyptian court sentenced the leader of the outlawed Muslim Brotherhood and 682 supporters to death on Monday, intensifying a crackdown on the movement that could trigger protests and political violence before an election next month.

The defendants were charged with crimes including inciting violence following the army overthrow of elected leader Mohamed Mursi, a senior Brotherhood member, in July after mass protests against his rule.

"These mass trials are clearly in breach of international human rights law," the EU's Catherine Ashton said in a statement.

"The exact charges against each defendant remain unclear, the proceedings lack the most basic standards of due process and the verdicts appear grossly disproportionate, failing short of complying with the principle of individual sentencing," she said.

Ashton said the EU was concerned about Egypt's compliance with its international human rights obligations, as well as "the seriousness of Egypt's transition towards democracy."

"The EU calls on the Egyptian authorities to immediately reverse this trend which jeopardizes any prospects for overcoming divisions within society and to ensure progress towards a truly democratic, stable and prosperous Egypt," she said. (Reporting by Justyna Pawlak; Editing by Cynthia Osterman)