Basque separatists have been blamed for a car bomb explosion in northern Spain which killed a leading political opponent and his bodyguard.

Fernando Buesa, head of the provincial chapter of the Basque Socialist Party, and a policeman died when the bomb was set off by remote control at about 1630 local time (1530 GMT) in the Basque capital, Vitoria.

No one has claimed responsibility for the blast, but police blamed Basque separatist organisation ETA, which announced an end to its 14-month unilateral ceasefire in early December.

ETA was also blamed for a car bomb blast in Madrid on 21 January that killed an army colonel.

Mr Buesa, whose party opposes Basque separatism, was walking near his home when the bomb detonated.

The 52-year-old father of three was among some 10,000 people who took part in an anti-ETA demonstration on Saturday in the nearby Basque city of San Sebastian.

Some said the ground shook when the car bomb detonated and a huge cloud of smoke enveloped the area. Three passers-by were slightly injured.

Thousands of people gathered in the centre of Vitoria on Tuesday evening to protest against the killing.

The protest was attended by the conservative Interior Minister Jaime Mayor Oreja, who branded the attack one of "relentless cruelty" and said the Spanish "must learn the macabre lesson from the way ETA is playing with us."

Earlier on Tuesday, Mr Oreja had warned that ETA was likely to carry out another attack before the general election.

The explosion happened on a university campus, about 200 yards from the headquarters of the president of the Basque regional government in northern Spain, Juan Jose Ibarretxe.

Mr Ibarretxe later broke off a parliamentary agreement with Eta's political wing.

Eta's ceasefire was called off after talks between the paramilitary group and the Spanish Government made little progress towards ending the 30-year conflict.

The past few weeks has seen growing tension in the area between those who support an independent Basque homeland and those who want to remain part of a united Spain.

Prime Minister Jose Maria Aznar pledged to continue his hardline stance against Basque separatism.

He said: "We are not only united in grief; we are united by the determination not to give in to barbarism, or to terror, or to terrorist blackmail.

"We have shown such unity, and we will continue to show it."

ETA has been blamed for about 800 deaths in its fight for Basque independence.

An estimated 1.1 million people marched in Madrid following the January bomb blast in an emotional call for peace.