Mr Zapatero (right) has been keeping a high profile abroad

BBC Madrid correspondent Flora Botsford says the visit has embarrassed the Spanish Government at a time when relations between the two countries are at a serious low point.

But the Moroccan state media described Mr Zapatero as the next leader of Spain and welcomed his visit as a gesture of goodwill.

Morocco withdrew its ambassador to Spain in October over a range of long-standing problems.

Spanish Prime Minister Jose Maria Aznar has accused his political rival of treachery, but Mr Zapatero says he will do everything for the good of Spain during his visit.

Disloyalty denied

The Spanish foreign ministry said Mr Zapatero had not sought advice about the visit and his trip amounted to "grave interference" in diplomatic affairs.

He is due to meet Moroccan Prime Minister Abderrahmane Youssoufi and Foreign Minister Mohammed Banaissa while an audience with King Mohammed VI has been arranged at the end of his visit on Tuesday.

Speaking in Spain before he left, Mr Zapatero rejected suggestions that he was being disloyal or trying to score points against the government.

Aznar: Accusation of disloyalty

He said that dialogue and co-operation between the two countries were important.

Spain and Morocco have disagreed over issues including fishing quotas, illegal immigration, drug smuggling and the political future of western Sahara - a former Spanish colony.

But observers say the underlying frustration for Rabat has been the negative portrayal of Morocco in the Spanish media.

The Moroccan media has also accused some Spanish journalists based in Rabat of spying for Spanish intelligence.

Mr Zapatero, who emerged out of relative obscurity to lead the Socialists last year, recently described Mr Aznar's foreign policy as a "resounding failure" which had "humiliated" Spain.

On the issue of agreeing fishing rights with Morocco, he said Spain had "ceased to be a power in fishing thanks to Mr Aznar's policy".

However, he has undertaken not to criticise his government during the visit.