The nine men, members of the Islamic Party (PAS), have already been held for two years under the draconian Internal Security Act.
"We are extending (their detention) based on the fact they still pose a security threat to the nation," a government official said on Tuesday.
The government accuses the PAS party of being extremist, but human rights groups have called for the nine men to be charged or released.
Detention orders can be renewed indefinitely under the security act, which the government says is necessary to combat "terrorism", but which critics say is used to muzzle the opposition.
The detainees include Nik Adli, a PAS member and son of PAS spiritual leader Nik Aziz Nik Mat, PAS youth wing leader Noorashid Sakip, and youth committee member Lutfi Ariffin.
New York-based pressure group Human Rights Watch has urged the government not to renew the detention orders, saying: "There are indications that the arrests were politically motivated to weaken PAS."
The government of Prime Minister Mahathir Muhammad alleges the men are connected with the Malaysian Militant Group (KMM), an Islamic movement accused of plotting the violent overthrow of the government.
But Human Rights Watch said: "Because of the secrecy of the process, no one outside the Malaysian security services knows what evidence there is against them."
A government official told Agence France Presse: "With ongoing investigations into terrorist networks in the region, we still have to ascertain their involvement with JI (Jamaah Islamiyah)."
Lack of evidence
The Jamaah Islamiyah group is blamed for a series of attacks in Asia.
Mahathir has been accused of
bowing to US pressure
"They may not be directly involved but there might be some overlaps," the official said.
Pressed on why Malaysia had failed to bring to court any of the 92 alleged Islamic group members it is holding in detention, the official said: "We are still trying to unearth and uncover the extent of their involvement and until then all of them are still security risks.
"If we let people out now, the puzzle will be incomplete. As soon as we get a clear picture with which we can bring them to court we will do so."
PAS president Abd al-Hadi Awang described the renewed detentions as "inhumane" and suggested the government had been influenced by the United States.
"It indicates the country has not gained real independence because it is bowing down to pressure from the US superpower who points fingers at Muslims alleging they are terrorists even though they cannot provide evidence," he said.
"We feel Malaysia was not acting on its own but due to pressure from the superpower. The whole allegation has not been put to the test at an open court. This is unfair."
He added: "We want to reiterate our stand against terrorism and acts of fanaticism. (But) any act against terrorists should be done through due process of law and we should not bow to demands from outside forces."