(Adds first accused denying involvement )

MADRID, Oct 15 (Reuters) - Thirty men accused of planning to blow up Spain's High Court in an al Qaeda suicide bomb attack went on trial in Madrid on Monday.

The men, mainly Moroccans and Algerians, were arrested in October 2004, seven months after a group of al Qaeda-inspired bombers killed 191 people in attacks on Madrid commuter trains.

Officials said the ringleaders planned to use a truck packed with half a tonne of explosives to carry out a suicide attack on the court, as well as aiming to bomb a train station and the headquarters of the conservative opposition Popular Party.

Prosecutors are asking for sentences of up to 46 years.

The first of the accused to appear, Mohamed Boukiri, denied involvement and said he was too heavily involved in his illegal drug habit to have taken part.

"I was just a junky," he told the court.

Many of the men, who are not known to be linked to the March 2004 train attacks, first met while serving prison sentences in Spain, prosecutors say.

Spanish security officials say that Islamist groups are the country's biggest security threat -- more dangerous than Basque nationalist guerrillas ETA.

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