Lieve Pellens, spokeswoman for Belgium's federal prosecutors, said those involved in the plot to free Trabelsi possessed an "extreme vision of Islam".
 
"They were planning to use weapons and explosives to free him ... These means could be employed for another use," she said.
 
"There is no guarantee that if these means are available, that they will not be used in another way during this period of the year when people get together" to celebrate Christmas, she said.
 
Trabelsi was arrested in Brussels in September 2001 and was jailed in 2004 for 10 years.
 
He has said that he planned to target a Belgian airbase believed to contain US armaments.
 
Risk prevention
 
"Belgium must adopt security measures after the discovery, during a police raid, of plans for a prison breakout," Alain Lefevre, interior ministry spokesman, said.
 
Officials held a news conference to announce 
security measures [Reuters]
"The justice authorities did not want to take any risk and wanted to foil any possible attack," he said, emphasising that there was no indication that an attack was being prepared.
 
Pellens said that the authorities were not aware of any other plot and urged Belgians not to panic.
 
Police patrols have been increased at Brussels' international airport, rail and metro networks in the Belgian capital and at commercial centres.
 
The enhanced security measures will be place until January 2, covering the busy Christmas and New Year period.
 
Belgium hosts Nato, European Union (EU) institutions and offices belonging to several multinational companies.