Zapatero will seek to balance his inexperience

The ceremony took place at the Royal Palace in Madrid.

One of Mr Zapatero's first acts as prime minister was to visit survivors of the Madrid train bombings and to lay flowers at Atocha station.

The 43-year-old leader is untested and relatively inexperienced but, say correspondents, his government will include a number of heavyweights.

Miguel Angel Moratinos, tipped to be the new foreign minister, was for seven years the European Union's envoy to the Middle East.

The BBC's Katya Adler in Madrid says he has already come to Mr Zapatero's diplomatic rescue on several occasions since last month's general election.

He smoothed feathers ruffled at home and abroad over the new prime minister's pledge to withdraw Spanish troops from Iraq if the United Nations does not get to play a greater role there.

I will govern for all with respect and with humility Jose Luis Rodriguez Zapatero Profile: Spain's new PM Key cabinet members

Another voice of experience in the Socialist government is the former EU commissioner for the economy, Pedro Solbes, set to become Spain's finance minister.

Jose Luis Rodriguez Zapatero is shortly expected to officially announce the full make-up of his first cabinet.

Bombings

Mr Zapatero has pledged to push for better equality between the sexes in Spain, starting with his own cabinet.

Half of the 16 ministerial posts will be occupied by women including the role of deputy prime minister.

The swearing-in ceremony was attended by Queen Sofia, Spain's chief justice and by the outgoing Prime Minister, Jose Maria Aznar.

Mr Zapatero took the oath standing before a table with a copy of a Bible and the Spanish constitution.

His party was swept to power in elections in mid-March, days after nearly 200 people were killed by bombs left on Madrid commuter trains.