PARIS -- Police dismantled a 25-kilogram bomb found near a police laboratory Monday, officials said, one day after the third terrorist bombing in three months injured four people and heightened fears of new attacks.


The explosive, hidden in a suspicious package, was placed in a public toilet at Place Charles Vallin in southern Paris, about two blocks from the Paris Police Laboratory where previous bombs have been analyzed, officials said on customary anonymity.


On Sunday, a homemade bomb hidden in a pressure cooker exploded under a fruit and vegetable stand in an outdoor market near Place de la Bastille, injuring four women and setting several food stalls ablaze.


There was no immediate claim of responsibility, but suspicion fell on Algerian Moslem fundamentalist extremists who have threatened France for its support of the military-backed government in Algiers. The Armed Islamic Group is suspected in the two other bombings, one of which killed seven people.


Although officials did not immediately announce a link to the bombings, Sunday's blast followed a similar pattern to the earlier attacks. The bomb was filled with nails and hexnuts and targeted a well-populated area.


The four victims suffered light leg and arm injuries as well as superficial burns, hospital officials said.


The casualties could have been worse. Authorities said it appeared that while the detonator went off, it failed to set off the rest of the bomb.


As a result of the new attacks, security on the first day of school Monday was the tightest since the last bombing wave that hit the capital in 1986, killing 13 people.


Only the parents of children in kindergarten were allowed on school grounds, all adults were closely screened and kids were told not to gather in front of their schools or leave their backpacks on the ground. All field trips were canceled until further notice, officials said.


Police mounted a nationwide effort to tighten security and search for suspects after the first bombing in a Paris regional subway train July 25 that killed seven people and injured 84. But even as police boosted patrols and searches and covered up trash cans in subway stations, another bomb exploded on a street near the Arc de Triomphe on Aug. 17, wounding 17 people, including 11 tourists.


On Aug. 26, authorities found a bomb planted on a high-speed train track north of Lyon as French vacationers were returning home. The bomb failed to go off.


Security was also tight in Brussels Monday as a trial of 13 suspected supporters of the Armed Islamic Group opened. About 200 police were to be deployed around the courthouse and patrols were stepped up in subways, train stations, the airport and embassies.