The centrists received only 10 of the 350 mandates as a result of Sunday's elections, losing 47 seats and taking sixth place.
According to Spain's El Mundo newspaper, Rivera admitted that the election results were poor and that he has no excuses. He noted that more than 4 million Spaniards voted for Citizens in the April general election, but only 1.6 million did so on Sunday.
"Spanish people chose Vox over the political center [parties]", the politician announced.
Rivera said that he was personally responsible for the failure of the party.
Citizens was founded in 2006, and Rivera has been its leader since nearly the date it was founded. In 2006, the party won three mandates in the Catalan regional elections. Over time, the Catalan party reached the national level, winning 40 seats in the Congress of Deputies, the lower house of the Spanish parliament, after the 2015 elections. The April 2019 Spanish parliamentary vote saw the party gain third place, just behind the Spanish Socialist Workers Party (PSOE) and People’s Party. Citizens makes up the largest fraction in the Parliament of Catalonia.
On Sunday, Spain held general elections for the fourth time in four years. The PSOE secured 120 seats in the parliament's lower house, while the conservative People's Party secured 88 seats. The right-wing Vox party secured 52 seats with 15 percent of the votes, while left-wing Unidas Podemos secured 35 seats with 12 percent.