Five small bombs exploded on highways near the Spanish capital Tuesday, but no one was injured and there was little damage, news reports said.
The daily El Mundo said police were searching for more bombs following on a warning call from the armed Basque group ETA.
Earlier Tuesday, police evacuated an airport in the northern city of Santander following a bomb warning made in the name of ETA, the Interior Ministry said.
The terminal and parking area were evacuated and searched shortly before noon after the Basque daily Gara said it had received a call from someone speaking on ETA’s behalf warning that one or more explosive devices would go off at the airport, a spokesman said.
ETA has claimed responsibility for more than 800 killings since the late 1960s in its campaign for a Basque state, but it has not killed anyone since 2003.
Spain on Tuesday commemorated the 27th anniversary of its constitution.