The Lebanese army and the security source said gunfire was exchanged.
Senior US official David Hale is expected in Beirut later on Thursday to stress the urgent need for financial and governance reforms, ending endemic corruption and bringing transparency.
Forming a government amid factional rifts has been daunting in the past.
But analysts said Hezbollah's role in the government's formation meant it might have difficulty convincing other states to provide urgently needed financial support.
A wave of protests erupted last October against a ruling elite accused of steering Lebanon towards its worst economic crisis since the 1975-1990 civil war.
If the matter had not been leaked to the media, he said, no one would have event notice, because the process had not gone on long enough to even constitute a delay.
After Hariri quit, protests had ebbed, roadblocks were lifted and banks reopened for the first time in two weeks on Friday.
It was the biggest counter protest to the massive wave of demonstrations that have gripped Lebanon since Oct. 17 and which have included Aoun's removal among its demands.
IDF, Hezbollah say drone wasn’t shot down
Banks, schools, and many businesses have shut their doors.