The suspects are eight Lebanese and one Palestinian, a senior security official said on condition of anonymity on Monday.
Security forces seized an unknown quantity of weapons with the suspects, the officials said.
One senior security official said: "The plot was at an early stage. We have smoke and we have fire but the details are not clear yet."
He said the Lebanese suspects were related to each other.
Hussein Rahal, a Hezbollah spokesman, said authorities had informed the group of the plot.
"We can confirm this," he said. "Lebanese authorities have informed us that they arrested a group accused of planning to assassinate Sayed Hassan Nasrallah."
Hezbollah's attacks were vital in ending Israel's 22-year occupation of south Lebanon in 2000. It is listed by Washington as a "terrorist organisation".
As-Safir, the Lebanese daily, reported on Monday that military intelligence had arrested nine alleged plotters last week after they were observed acting suspiciously near Hezbollah's headquarters in the southern Beirut district of Haret Horeik.
The military is looking for their accomplices, the paper said.
Another security source said the suspects were planning to assassinate Nasrallah on April 28, the day when he was expected to attend another round of national dialogue talks with other leaders at the Lebanese parliament.
Lebanese leaders have failed
to bridge gaps on key issues
The talks have failed to bridge gaps on key issues dividing the country, including the domestic and international calls for Hezbollah to disarm.
Hezbollah, also a staunch ally of Damascus, says it will not disarm even if Israel withdraws from the occupied Shebaa Farms. The UN says the border area is Israeli-occupied Syrian land unless Beirut and Damascus amend their border
Nasrallah's movements in Lebanon are known to be limited and he is usually accompanied by heavy security.
His predecessor, Sayyed Abbas al-Mousawi, was killed in an Israeli raid in 1992.
The suspects were questioned by military intelligence and then handed to a military magistrate, a security official said.
A string of bombings and political assassinations has rocked Lebanon since the February 14, 2005 killing of Rafik al-Hariri, the former prime minister.