All eyes will be on Mr Zapatero and his new cabinet

The 16 ministers were sworn in at the Madrid palace of Spain's King Juan Carlos where Mr Zapatero took his oath the previous day.

An equal number of men and women have cabinet rank in the new government.

Correspondents say Mr Zapatero has picked several political heavyweights to offset his own relative inexperience in high office.

The new ministers swore loyalty to the crown and the constitution at the Royal Palace.

The BBC's Katya Adler in Madrid says this minority government has big challenges ahead, having already been branded "weak and unstable" by the opposition conservative leader.

Mr Zapatero's Socialists won last month's elections without a full majority, but have not sought allies for a coalition - apparently, say Spanish media, because the new leader does not want to see his plans for sweeping reform compromised.

Terror frontline

Big names in the new government include Miguel Angel Moratinos, a former diplomat and EU envoy to the Middle East, who has taken charge of the foreign ministry.

Socialist party chief Jose Bono becomes defence minister, while Pedro Solbes - a man credited with edging Spain towards the euro - becomes finance minister.

Mr Moratinos is expected to travel shortly to Washington to discuss Spain's stance in the US-led wars in Afghanistan and Iraq.

Mr Zapatero is widely expected to deliver on a pre-election promise to withdraw Spanish troops from Iraq unless the UN is given a bigger role there.

But he has promised to balance this by sending more soldiers to Afghanistan.

Days before last month's election, Spain was hit by bombs planted by suspected Islamic militants, leaving nearly 200 people dead in Madrid.

The previous government's management of the bomb attacks - and its support for the war in Iraq - were widely thought to have caused its election downfall.