An explosion hit a railway station in the town of Berriz in northern Spain, claiming no casualties but causing major material damage, the Basque regional interior ministry said.
Two hours later, Molotov cocktails were thrown at the station in the neighbouring commune of Amorebieta, but again no one was hurt, a ministry spokesman told AFP.
"There was an explosion at the Berriz Euskotren (regional train) station. There is very serious material damage and no one was injured," the spokesman said.
Police were on the scene "to determine whether it was an act of urban violence or a terrorist attack," he said, adding that the blast had not been preceded by a telephone warning as is common practice by the armed Basque separatist group ETA.
The Euskotren station at Amorebieta was the next target with two Molotov cocktails lobbed at the front of the building damaging the ticket offices and machines.
Berriz and Amorebieta are situated east of Bilbao, near the highway linking the Basque cities of Bilbao and San Sebastian.
Acts of urban violence are common in the Basque country, often taking the form of Molotov cocktails thrown at official buildings, stations or automatic teller machines.
They are attributed to young radical supporters of Basque independence considered by police as "support groups" for ETA and a major source of its recruits.
ETA, listed as a terrorist organisation by the European Union and the United States, is considered responsible for 824 deaths in its 40-year-long violent campaign for an independent Basque Country, which straddles northern Spain and southwest France.