Scenes of jubilation broke out at the offices of the protest group Families of the Disappeared.

"It is a transcendental moment for all of humanity," Viviana Diaz, the vice-president of the group representing families who lost members to the military regime, said.

The BBC's Ben Brown in Santiago: Dramatic scenes at both pro- and anti-Pinochet camps

Horns blared in the centre of the Chilean capital and nearby neighbourhoods as drivers celebrated.

But supporters of the general violently rejected the ruling, venting their anger on reporters in Santiago.

Pro-Pinochet protesters pushed, jeered and shouted insults at representatives of the national and international media covering reactions to the verdict.

The reaction in in Madrid's Puerto del Sol central square

Chilean television switched away from the violent scenes for several minutes, and then returned to broadcast an interview with General Pinochet's son, also named Augusto Pinochet.

"My father has received a sadistic and cruel blow on his (83rd) birthday that goes beyond the rights of mankind," he shouted.

Members of the government appealed for calm, fearing a repeat of the violent demonstrations which occured following General Pinochet's arrest last month.

The BBC's James Reynolds in Santiago: Authorities braced for trouble

BBC correspondent Ben Brown, in Santiago, says the mood turned ugly in the city hours after the ruling was announced.

Riot police were called in to clear anti-Pinochet demonstrators from the streets, using water cannon and tear gas.

Chilean President Eduardo Frei said Chile would contest the ruling and reiterated Chile's claim that General Pinochet, a senator for life, enjoys diplomatic immunity.

Mr Frei said he would send a protest message to London and ordered Foreign Minister Jose Miguel Insulza to fly to Europe to back up his government's stance.

Madrid wary

Anti-Pinochet protesters in Madrid greeted the Law Lords' decision ecstatically.

The BBC's Daniel Schweimler reports from the celebrations in Madrid

"I'm very deeply moved," said Carlos Slepoy, a prominent human rights lawyer who has worked for years on behalf of victims' families. "It is an irreversible step towards the prosecution of crimes against humanity."

Isabel Allende, the daughter of former Chilean President Salvador Allende, welcomed the news.

Isabel Allende says her father has at last been vindicated

"There was always hope at the bottom of my heart ... On 11th September (1973, the day of Pinochet's coup) my father behaved with dignity and courage. He said the events would be punished morally as a crime of treason," she said, speaking in Madrid.

"I think that in many ways his prediction has come true 25 years on."

The BBC's correspondent in Madrid Daniel Schweimler says that while opponents of General Pinochet in Spain are celebrating, the Spanish government has sought all along to keep its distance from the case.

Madrid is sensitive about its relations with Chile, which have been very close, and is also concerned for the safety of some 40,000 Spaniards living in Chile.