PARIS (Reuters) - Three suspected Islamist militants arrested in southern France appeared to be planning an attack in the days ahead, the Paris prosecutor said on Monday, the anniversary of an al Qaeda-inspired shooting that rocked France.

Police found weapons and explosives at the home of one of the suspects in the town of Marignane, near Marseille, and intercepted communications between the men suggested they were close to going into action, prosecutor Francois Molins said.

The three men, who were taken in for questioning last week with a fourth man who was later released, were to be placed under formal investigation later on Monday.

"The investigation showed we were faced with a veritable laboratory for making improvised explosive devices and, even if we had no evidence of a precise plan, the messages exchanged by the participants suggested the timetable could be accelerated," Molins told reporters.

The timing of the arrests was poignant, coming exactly a year after 23-year-old gunman Mohamed Merah began a rampage that killed three Jewish children, a rabbi and three soldiers in the southern city of Toulouse. He was subsequently tracked down and killed in a shootout with police.

Speaking as Defence Minister Le Drian led a memorial ceremony in Toulouse for Merah's first victim and posthumously awarded him the prestigious "Legion d'Honneur" medal, Molins said the arrested men, in their 20s, wanted to emulate Merah.

"It was clear they were training themselves in making explosives based on a jihadist radicalization, a glorification of Mohamed Merah, and an affirmed desire to go into action."

France, which centralizes its terror investigations in Paris, is on high alert for any home-grown militant activity since its military intervention against Islamist fighters in Mali in January prompted threats of retaliatory action.

(Reporting by Catherine Bremer and Patrick Vignal; editing by Mark Heinrich)