Siniora on Tuesday confirmed that the three former security chiefs had been detained for questioning and that the guards commander had been summoned to appear before the UN investigation.
The prime minister spoke hours after police detained at their homes Major-General Jamil Sayyed, the former chief of General Security; Major-General Ali Hajj, the former director-general of the Internal Security Forces; and Brigadier-General Raymond Azar, the former director-general of military intelligence.
Presidential guard chief Mustafa Hamdan turned himself in to United Nations investigators after a warrant was issued, and former pro-Damascus minister Nasser Qandil broke off a visit to the Syrian capital to appear before the inquiry commission.
They are the first major figures brought in for questioning in the investigation into al-Hariri's killing, which plunged Lebanon into turmoil and increased the pressure on Syria to pull out its troops in April.
Hamdan(R) has appeared before
the UN investigation
The five men are to be questioned and can be held for up to four days, after which they would be released or charged under the Lebanese justice system, legal sources said.
The chief UN investigator, Detlev Mehlis, separately met the prime minister and justice minister earlier on Tuesday.
The Justice Ministry statement said the UN investigation had been granted permission to use the police to "carry out raids, searches and escorting of persons for questioning".
Lebanese security officials reported that former justice minister Adnan Addoum had also been summoned to the UN investigation, but the ministry statement did not mention its former minister.
Aljazeera correspondent Ghassan bin Jiddo in Beirut said Hamdan is considered to be the most informed among the detained security officials.
Siniora confirmed the arrests of
the former security chiefs
UN investigators spent nine hours interviewing Hamdan, bin Jiddo said.
The arrest warrant was issued by the international investigation panel, not by the Lebanese security authorities, who only executed the order, the correspondent added.
Asked if that meant new evidence had been found in al-Hariri's assassination case, bin Jiddo said UN officials declined to comment.
President Emile Lahoud played down the questioning of the five security officials, in a meeting with a US congressional delegation, Lahoud's office said in a statement.
"Nobody has been arrested. It's just part of the ongoing inquiry, but the media are talking of arrests for domestic political reasons," the president said.
But legislator Walid Jumblatt, the Druze leader, said Lahoud was likely to have to stand down as the truth unravels about the assassination.
"I think the countdown has started and I expect the fall of important figures in Lebanon and abroad," he told Radio-Orient. "I don't think he will be able to continue as president of Lebanon."
The detainees and the guards commander have previously been questioned by Mehlis, the German prosecutor who is leading the UN Security Council-mandated investigation into the assassination.