Carles Puigdemont sworn in as president for Catalonia autonomous region on Sunday evening is known as a separatist who aims to break off the region from Spain over the next 18 months.
After months of hard negotiations in Catalonia, parties agreed to nominate Puigdemont as the leader who could unite the separatist movement.
Puigdemont, former mayor of Girona, replaced Artur Mas, who was the leader of the Catalan parliament, which is expected to accelerate the push for unilateral split with Spain.
Puigdemont was voted in as Catalonia’s 130th president in a 70-63 vote, with two abstentions in the 135-seat chamber.
Catalonia's parties had until Monday to agree on a new leader or new regional elections would had to be called.
Under the separatists' 18-month "roadmap," Catalan authorities will approve their own constitution and begin building institutions necessary for an independent state such as an army, central bank and judicial system.
"We begin an extremely important process, unparalleled in our recent history, to create the Catalonia that we want, to collectively build a new country," Puigdemont told the Catalan parliament.
He stressed that Catalonia must negotiate with the Spanish government, the European Union and the international community to reach its goal.
Spanish central government refuses such negotiations and disallowed a referendum in 2014, saying it would contravene Spanish constitution.
Acting Spanish Prime Minister Mariano Rajoy said on Sunday he would block any unilateral move from the new Catalan government and had instructed all government officials to enforce the law.
"I will defend and preserve democracy all over Spain. I will defend the sovereignty of the Spanish people," he said in a news conference.
Rajoy’s People’s Party won the last general elections on December 20 but lost the majority in Spanish Parliament and the country has been in stalemate since then.
Rajoy said Spain’s courts had already ruled against Catalonia’s pro-independence move, declaring it unlawful.
“I want to make it clear that the Spaniards can be calm,” Rajoy said.
Rajoy added that he has got support from Socialist party leader Pedro Sanchez and newcomer centrist party Ciudadanos' leader Albert Rivera to form a German-style “grand coalition” to deal with the separatist movement.
The agreement between Junts pel Si (Together for Yes), a coalition of the centre-right CDC party and leftist ERC, and the anti-capitalist minority partner CUP gives the separatist bloc a slim majority in the 135-seat Catalan parliament.
On top of Junts pel Si's 62 seats, the CUP will add two seats of its own and its other eight delegates agreed to not oppose the government on issues of independence. Two of the eight CUP delegates abstained in the vote for Puigdemont.
According to opinion polls in November 2015, 7.5 million people in Catalonia region say they want to remain part of Spain but with greater autonomy on issues such as tax.