MADRID - Upstart parties are expected to make strong headway in Spanish regional elections on Sunday, ushering in an era of coalition and compromise and potentially dampening Prime Minister Mariano Rajoy's prospects of winning a second term later this year.

Rajoy's conservative People's Party (PP) is likely to win most votes in elections taking place in 13 of Spain's 17 regions and more than 8,000 towns and cities. Opinion polls, however, indicate the PP may find it hard to govern in most regions and in some of the biggest cities, including Madrid and Barcelona, as leftist Podemos ("We Can") and centre-right Ciudadanos ("Citizen") split the vote and could join forces with other parties against the PP. Podemos has pledged to overhaul the two-party system that has dominated the political scene for 40 years and inject more transparency.

 and accountability into Spanish politics. They are both expected to increase their support base on Sunday, especially in the Madrid and Valencia regions, two PP bastions since the mid-1990s which will be closely watched as a bellwether for the national election, due in November.

Yet, with final opinion polls showing 30 to 45 percent of voters were still undecided ahead of the vote, and with analysts expecting turnout to rise due to renewed interest in politics, Sunday's outcome could deliver surprises. Sara, a 34-year-old sales representative from Madrid who declined to give her surname, said she had abstained in the last European and general elections but had been motivated this time by the new options available, such as the Ahora Madrid ("Madrid Now") platform formed by Podemos and other leftist movements.

"They offer a change by the people and for the people ... I want to believe that they will make a real difference," she said, after casting her ballot in the Tribunal neighbourhood in central Madrid. Her friend Roberto Diaz, a 29-year-old school teacher from Madrid, also voted for Ahora Madrid and said he hoped new parties would bring a more direct form of democracy as well as social change. In the past he had always voted for the Izquierda Unida, a platform of small far-left parties.

Analysts expect that at least three political groups will be needed to form majorities in 12 of the 13 regions and in all the main cities, a brand new landscape in a country with virtually no tradition of coalition government at national or regional level as the PP and the centre-left Socialists have alternated in power since the end of dictatorship in the 1970s. In Barcelona, where seven parties are expected to gain seats in the city council, several candidates have already warned that a new election may have to be called.

There is already such a stalemate in Spain's most populous region, Andalusia. A new parliament was elected in March but no government has yet been formed because the winning Socialists have so far failed to convince Podemos, Ciudadanos and the PP to back a minority government.