When abstention (~55%) is considered, their share of the popular support is barely above 20%.
|Electoral results in Spain|
(source: Kaos en la Red)
The other side of the coin is the notable rise of the "radical left" lists: United Left and Podemos, who cater a very similar niche. Add to them the more modest achievement of Equo (European Spring coalition) of "green" ideology.
Podemos (We Can) was the great surprise. The party, which emanates from sectors of the indignados movement, has gathered much of the discontent about the lack of real (participative) democracy, which was the keystone of their program and political philosophy. They collected 1.2 million votes starting from zero and, while opinion polls suggested that they would get one MEP, they finally got five.
United Left (La Izquierda Plural in these elections), coalition formed around the Communist Party and akin to Greek Syriza, the French Front de Gauche or the German Die Linke, also gained one million votes over their previous base, obtaining 6 MEPs (they had only 2).
It seems apparent that both forces will have to negotiate some sort of cooperation in order to maximize their chances in the 2015 state elections. Their electoral programs are similar but there is some sort of rivalry between a weathered aparatchkik with almost a century of political history and these startups which appeal to popular assemblies.
Some maverick reactionary forces also got some support: UPyD, often accused of flirting with fascism and clearly extremist Spanish nationalist in their ideology, gained 3 MEPs (had one before). The other far right force that collected important support is the xenophobic Catalan unionist list Ciutadans, which managed to get 2 MEPs.
The other forces gaining (mostly pointless) Euro-representation are independentist forces of Catalonia, the Basque Country and Galicia, which ran in three state-wide coalitions and got 6 MEPs.
For details on the Basque Country see here.
Sources: Kaos, La Marea (link 1, link 2).