SPANISH opposition parties have launched a fierce campaign to end the conservative government of Mariano Rajoy after courts ruled his Popular Party profited from a large kickbacks-for-contracts scheme.

The Socialist opposition announced a vote of no confidence against the prime minister with the backing of anti-establishment and left-wing parties, while the pro-business Ciudadanos (Citizens) – which had supported the conservative minority government until now – urged Rajoy to call a fresh election.

He ruled out stepping down, saying he was committed to remaining in power until the end of his term in 2020.

"All this is nonsense," the 63-year old conservative politician told reporters in a televised press conference.

He called the opposition's move "opportunist", and said the no-confidence vote "goes against the stability in Spain, damages the economic recovery, introduces uncertainty and goes against the interests of all citizens".

The moves followed the conviction of 29 business people and former Popular Party officials for crimes including fraud, tax evasion and money laundering over an illegal 1999-2005 scheme that, according to Thursday's court decision, helped fund the ruling party.

The ruling triggered "social indignation" that has put Spain "in a situation of an extreme institutional crisis", Socialist leader Pedro Sanchez said.

"There is only one person responsible for the political disaffection," he told reporters. "That person is called Mariano Rajoy."

Parliamentary rules in Spain require a proposal of an alternative prime minister in a no-confidence vote.

Sanchez said he will call elections if he wins the vote. Meanwhile, the popular centre-right Ciudadanos (Citizens) party urged Rajoy to call elections or face a separate no confidence vote.

"We need a clean and strong government to confront the separatist defiance (in Catalonia)," said its leader Albert Rivera.

No date has been set for the vote, but it could be as early as next week according to parliamentary rules. The Socialists will need at least 176 of the 350 votes in the national congress, where they only control 84 seats against 136 held by the Popular Party.

Although the Socialists have enlisted the backing of 71 left-wing and anti-establishment legislators, they would need the 32 extra votes of the pro-business Ciudadanos, which is reluctant to side with the left-wing or the Catalan separatists.

A successful motion of no confidence is unlikely at this stage, said Antonio Barroso, an analyst with London-based consultancy firm Teneo Intelligence. "Opposition parties will likely engage in a blame game rather than effectively coordinate to get rid of Rajoy," he said.

Today's political jockeying comes after a rollercoaster ride this week for Rajoy's party, whose win on Wednesday of a key approval for the 2018 national budget had secured, in theory, enough breathing space for him to survive until the end of the term in 2020.

But the setback came less than 24 hours later in the form of a 1687-page ruling on the so-called Gurtel case, considered one of the gravest corruption episodes in Spain's modern history.

The judges issued prison sentences totalling 351 years and a €245,000 fine (£214,000) for the conservative party in power, which the ruling describes as a "profit-seeking participant" in the scheme.

The verdict also considered that a network involving companies and party officials was established to arrange travel and organise events for PP in exchange for public contracts.

In some of the most damaging parts of the ruling, the judges said PP ran a slush fund at least until 2008, and questioned the credibility of Rajoy when he denied knowing the scheme was in place during a court hearing where the prime minister testified as a witness.