UN investigators are questioning five senior pro-Syria figures in Lebanon as suspects in the assassination of former Prime Minister Rafik Hariri.
The arrests are the first since the death of Mr Hariri in February
Three, including former head of general security Jamil al-Sayyed, were arrested and two others handed themselves in.
All five have close ties to Syria, which was widely blamed for the blast.
Syria has denied any role but has been criticised for hindering the UN probe into the bombing last February, in which 21 people died.
The chief UN investigator, German prosecutor Detlev Mehlis, is due to report his findings to the Security Council in the next few weeks.
BBC correspondent Kim Ghattas says the detentions constitute the first major development in the investigation into Mr Hariri's killing.
In addition to Mr Sayyed, the former internal security forces head Ali al-Hajj and former military intelligence head Raymond Azar were seized in early morning raids on Tuesday.
Mustafa Hamdan, the head of the presidential guard, later turned himself in to the UN officials.
A UN team is investigating Rafik Hariri's death last February
Nasser Qandil, a former member of parliament and staunch Syrian ally, returned from a visit to Damascus to face the investigators.
Lebanese Prime Minister Fouad Siniora told reporters that Mr Mehlis had briefed him on the investigation, and he decided to summon the four security chiefs "in order to question them as suspects".
Mr Mehlis has previously interrogated all the men except Mr Qandil.
Saad Hariri, son of the assassinated prime minister and a prominent MP, told the al-Arabiya TV channel from Paris: "The detentions in Beirut are the beginning of justice.
"This is a start... There will be more detentions. The most important thing is to know who killed Rafik Hariri."
Lebanon's President Emile Lahoud has played down the detentions and said the five remained innocent until proven otherwise.
Speaking to a visiting US congressional delegation, the pro-Syrian president defended Mr Hamdan as a loyal army officer who had saved his life in 1983 during Lebanon's civil war.
Mr Lahoud has himself been accused by anti-Syrian opposition politicians of complicity in Mr Hariri's death, and has resisted pressure to stand down.
Mr Sayyed was widely seen as Lebanon's most powerful security figure between the end of the civil war in 1990 and the withdrawal of Syrian forces earlier this year.
He and the others resigned earlier this year after huge anti-Syrian demonstrations following Mr Hariri's assassination.
The UN team has no power to arrest or charge suspects, but has a co-operation agreement with the Lebanese authorities and can request action through the internal security services.
Under Lebanese law, the four men can be questioned for 48 hours, after which they should be charged or released.