President Michel Sleiman called Monday on the Iranian embassy and Lebanon's powerful Shiite movement to help investigate the killing of an anti-Hezbollah protester outside the mission.
Hashim al-Salman, a Shiite, was shot in the back in an attack Sunday near the embassy in Beirut on demonstrators protesting the Iranian-backed Hezbollah's military involvement in the Syrian conflict.
Sleiman's office urged Lebanon's judicial and security authorities "to work quickly to uncover the circumstances of the incident yesterday in front of the Iranian embassy, and to arrest those behind it".
The president, in a statement, "emphasised the need for civilians and parties in the neighbourhood, especially Hezbollah and the Iranian embassy, to help... uncover all the details" around Salman's death.
Cooperation from Hezbollah and the Iranian embassy's would be key to "prevent such incidents from recurring", he said.
The anti-Hezbollah protest took place in the Bir Hassan neighbourhood of Beirut, a short distance from a key Hezbollah stronghold.
Salman was a student leader in a Shiite party which opposes Hezbollah and its main allies, Syrian President Bashar al-Assad's regime and Iran.
Salman, other protesters and journalists at the scene were assaulted by men wielding sticks as they approached the Iranian embassy. The Lebanese army said an unidentified man then fired live rounds from a revolver.
Salman was critically injured and later died of his wounds.
Prominent Shiite politician Ahmad al-Assaad accused Hezbollah of being behind Salman's killing.
Tensions in Lebanon have soared as Syria's conflict has increasingly spilled over into the small Mediterranean country.
Damascus dominated Lebanon for nearly three decades until 2005, and it still exerts significant influence on the country through its Hezbollah-led allies.