By Kerin Hope in Athens and James Fontanella-Khan in Brussels, Financial Times
In another closely watched development, the neo-Nazi Golden Dawn party, whose leader is in jail awaiting trial on charges of running a criminal organisation, grabbed third place, edging out the PanHellenic Socialist Movement (Pasok), the junior coalition partner in the government.
The explosive rise of Syriza and other once-marginal “protest” parties reflects voters’ anger over successive austerity programmes and frustration that they are being implemented by the same political elite that brought about Greece’s economic collapse.
Yet the election result – while a landmark for Syriza – was not expected to destabilise the government of premier Antonis Samaras, as the two coalition partners together captured a bigger share of the vote than Syriza.
“Voters sent the government a strong message but they rejected the overthrow that Syriza wanted,” Mr Samaras said in a televised statement conceding defeat.
Alexis Tsipras, the Syriza leader, claimed a “historic victory” for his party, an unwieldy group of factions ranging from former communists to social democrats, among them prominent defectors from Pasok.
Mr Tsipras has pledged to tear up Greece’s “barbarous” bailout agreement if he comes to power – a prospect that has unnerved the country’s eurozone partners. He said his election win meant the government no longer had the right to negotiate with international lenders on key issues such as a restructuring of the public debt, or to impose further austerity measures.
New Democracy-backed candidates performed well in a second round of local and regional elections also held on Sunday, winning run-off mayoral contests in Athens and Salonica, the country’s second-largest city, as well as sweeping 11 out of 13 regional governors’ posts.
Yet Syriza claimed a critical race for governor of the Attica region surrounding the capital, which is home to about one-third of the country’s population.
Rena Dourou, the shadow foreign minister, won a narrow victory over the incumbent, Yiannis Sgouros of Pasok with 50.4 per cent of the vote.
“There will be no longer be first and second-class citizens in this region, we’ll all go forward together,” Ms Dourou told a crowd of cheering supporters, pledging to alleviate poverty and social exclusion.
● In Italy, the first exit polls indicated victory for prime minister Matteo Renzi’s centre-left Democratic party, followed by the anti-establishment Five Star Movement. Silvio Berlusconi’s centre-right Forza Italia trailing well behind in third place.
The Democrats were seen winning with 29.5-32.5 per cent of the vote, with Beppe Grillo’s eurosceptic Five Star Movement taking 25-28 per cent. Forza Italia was predicted to come third with 18-20 per cent.
A success for Mr Renzi would help give Italy’s reformist and pro-Europe prime minister the electoral legitimacy he lacks after taking office in February by ousting his predecessor in an internal party leadership vote.
● Spanish voters turned their backs on the country’s established parties in unprecedented numbers, with a newly-formed leftwing party, Podemos, surprising pollsters by winning five of Spain’s 54 seats in the new parliament.
The new party, Podemos, was created only this year. It has its roots in the 15M anti-government protest movement that rose to prominence during the financial crisis.
The centre-right Popular party of Mariano Rajoy, the prime minister, saw its number of seats fall from 24 to 16, but still emerged as the biggest party.
Ukip and National Front aside, several other eurosceptic parties did well. The rightwing populist Danish People’s party won the largest share of votes in Denmark with 26.5 per cent, while the extreme right Jobbik in Hungary got 14.7 per cent.
Additional reporting by Guy Dinmore in Rome and Tobias Buck in Madrid