Spanish Socialist Pedro Sánchez On Saturday has been sworn in as the country's new prime minister by King Felipe after the ousting of conservative Mariano Rajoy.
Mr Sánchez, who is an atheist, took the oath to protect the constitution without a bible or crucifix - a first in Spain's modern history.
He plans to see out the remaining two years of the parliamentary term, reports BBC.
The Socialist leader won the support of six other parties to remove Mr Rajoy over a massive corruption scandal.
As Spain's new prime minister, whose party only has a quarter of the seats in parliament, he now has to decide who to include in his cabinet and is expected to name them next week.
In a brief ceremony at the royal residence in Madrid on Saturday, Mr Sánchez, 46, promised to "faithfully fulfil" his duties "with conscience and honour, with loyalty to the king, and to guard and have guarded the constitution as a fundamental state rule".
Mr Sánchez brought about the downfall of his predecessor Mr Rajoy by filing a no confidence motion in parliament following a scandal centred on a secret campaign fund that the conservative People's Party (PP) ran from 1999 until 2005.
Mr Rajoy is the first prime minister in modern Spanish history to be defeated in a no-confidence motion.
The new Catalan President Quim Torra, who is a close ally of ousted separatist leader Carles Puigdemont, said that he was committed to continuing the drive for the region's independence from Spain.