BERLIN - The three al-Qaeda suspects arrested on Friday in Germany had been plotting a bomb attack against a gathering of people in the country, German authorities said Saturday.

Police had monitored the three suspects for six months and found that they were testing explosive devices in recent days, Jrg Ziercke, president of the Federal Office of Criminal Investigation (BKA), said in a press conference in the southern city of Karlsruhe.

To prevent the possible attack, German police stormed the three 's bases on Friday morning in the western city of Bochum and Dusseldorf, Ziercke added.

Rainer Griesbaum, deputy federal prosecutor, told reporters that the plot, which was ordered by some high-ranking al-Qaeda leaders, had been under preparation for months. However, the three were "still in the experimental phase" and had not picked concrete targets.

Griesbaum said the principal suspect, 29-year-old Abdeladim El- K. with Moroccan nationality, received orders from "a high-ranking Al-Qaeda figure in spring 2010 to launch a bomb attack in Germany. "

Prosecutors said that El-K., once living in Bochum as a college student, attended an al-Qaeda training camp along the Afghan- Pakistan border in early 2010 and returned to Germany to prepare the plot in May of that year.

The ringleader had been living in Germany illegally since last November with no valid visa, and his accomplices were seven to eight people and possibly more, Ziercke said.

All three suspects were sent to the Federal Court of Justice on Saturday with charges of plotting a terror attack and being members of a terrorist organization.

German Interior Minister Hans-Peter Friedrich said that the arrests have eliminated "a concrete and imminent threat of international terrorism," but Germans should still keep alert on possible terrorist attacks and potential extremists.

Ziercke also warned that threats of terror attack on German soil still remained. Authorities estimated that some 130 potential extremists in Germany might plan to carry out such attacks.

Germany strengthened its security level at major airports, railway stations and tourism sites since November 2010, after receiving intelligence that al-Qaeda might launch Mumbai-style attacks in Europe, especially in Britain, France and Germany. In February, the government said the security alert would be gradually scaled back.

In November 2008, ten well-armed militants struck two hotels and seized hostages in India's largest city Mumbai, killing 166 people and injuring more than 300.