Police here arrested a fugitive Italian extremist yesterday who they said fits the description of a man seen with a suitcase at the Bologna, Italy, railroad station moments before a bomb exploded, killing 76 persons.

Marco Affatigato was arrested because he had false identity papers. He was been sought by Italian authorities in connection with several criminal charges in 1978.

At least three other suspected rightist extremists were arrested with Affatigato, police said. It was not known whether they also were suspects in the Bologna case.

French and Italian authorities said Affatigato resembles a composite police picture drawn up with the help of survivors, who said a man carrying a suitcase had been in the second-class waiting room shortly before the explosion. Investigators later found fragments of explosives in the room.

Affatigato was sentenced in absentia last month by a court in Pisa, Italy, to 3 1/2 years in prison for helping in the escape of Mario Tuti, a rightist extremist.

Tuti was later arrested in France.

Just two hours before the explosion on Saturday, Tuti was indicted for the 1974 bombing of a train outside Bologna. Twelve persons were killed in that explosion.

Affatigato has also been sought on a warrant charging him with trying to reorganize the outlawed Fascist Party.

Meanwhile, Italy declared a day of mourning for the 76 victims of the explosion last week.

Thousands of persons gathered in Bologna's main square for a memorial service. President Sandro Pertini and Prime Minister Francesco Cossiga attended the funeral mass.

Shortly before the mass, the coffins of all but eight victims were removed from the church for private burial. Many of the victims' families said they did not want anything to do with a funeral arranged by the government because authorities had been unable to stop extremist violence.