Libyan militants have called for the kidnapping of US citizens in Tripoli and for attacks on gas pipelines, ships and planes to avenge the capture of a senior al-Qaeda figure by US special forces in Libya last week.
Nazih al-Ragye, better known as Anas al-Liby, is a suspect in the 1998 bombings of the US embassies in Kenya and Tanzania that killed 224 civilians.
He was snatched on the streets of Tripoli on Saturday and is being held aboard a ship in the Mediterranean Sea, US officials said.
Mr Liby is wanted by the FBI, which gives his age as 49 and had offered a $5m reward for help in capturing him.
He was indicted in 2000 along with 20 other al-Qaeda suspects, including Osama bin Laden and current global leader of the militant network, Ayman al-Zawahri.
Messages posted by Libyan jihadists on the internet and monitored by the SITE service included a Facebook page that told Libyans to close off entrances and exits to the capital and kidnap citizens of the US and its allies in order to use them to bargain for the release of imprisoned militants.
It also urged them to damage pipelines exporting gas to Europe, and target ships and planes.
In another message posted on forums and social media, a group called "the Revolutionaries of Benghazi - al-Bayda, Derna" condemned the al-Qaeda leader's capture.
It accused Libya's leaders of having prior knowledge of the operation, although Prime Minister Ali Zeidan said at the weekend that the government had asked the US to explain the raid.
The group vowed to fight "everyone who betrayed his country and involved himself in this conspiracy. We say that this shameful act will cost the Libyan government a lot and it will be as you will see and not as you hear".
Since Muammar Gaddafi's fall, Islamist militants, including groups linked to al-Qaeda, have used Libya to smuggle out weapons and as a base for fighters.
North Africa is home to al-Qaeda in the Islamic Maghreb and other Islamist affiliates who either co-operate with the network or sympathise with its ideology.