The rally, as with others throughout the country, had been organised by NGOs, trade unions and the prime minister's own Socialist Workers' Party, the PSOE.

At a similar demonstration in Madrid, the main banner read "We will stop the genocide," while some protesters later threw stones at the Israeli embassy.

The deputy general secretary of the PSOE, Jose Blanco, said that it had put "objections" to the wording of the banner, but that he "had to be present" nevertheless.

On Sunday, the Israeli embassy issued a statement regretting the presence of members of the governing party at anti-war protests and saying that people demonstrating applied a "double standard" to Israel.

Israel's ambassador to Spain, Rafael Schutz, on Monday criticised Mr Zapatero's words as "unfair," saying in an interview with Cadena Ser: "Nobody says anything" in Spain about rocket attacks from Hamas.

Mr Schutz however rejected the idea that the prime minister's words would result in a diplomatic conflict between the two countries.

The ambassador said that he wanted to separate relations with the government from what had happened during the demonstration in Madrid.

"Demonstrations are one thing and dialogue is another," he said, adding that he recognised that Mr Zapatero had maintained an "authentic preoccupation" with the situation prior to the launch of the offensive, and that he "truly wants to contribute a solution" to the conflict.

Spanish foreign minister Miguel Angel Moratinos, a former European Union Middle East envoy, landed in the region on Monday to push foward the Franco-Egyptian ceasefire proposal.

He was to speak to his Israeli counterpart, Tzipi Livni, in Tel Aviv, and later head to Cairo, Damascus and the West Bank.