EU foreign policy chief Catherine Ashton "strongly" condemned the car bomb attack in central Beirut on Friday which killed eight people including a top intelligence official, Brigadier General Wissam El-Hassan, linked to the anti-Syrian camp in Lebanon.
Friday's rush-hour bombing occurred in Ashrafieh, a predominantly Christian neighbourhood of the Lebanese capital.
"This shocking attack stands in stark contrast to the recent efforts to rebuild this country, guarantee its stability, consolidate the sense of national unity and promote a culture of dialogue,"Ashton said in her statement.
"The perpetrators of this crime must be pursued and brought to justice. I call upon all Lebanese toremain calm and to ensure that this attack does not destabilise the country."
Ashton said the EU "will continue giving its full support to the efforts undertaken to advance security and unity in Lebanon, which are essential to the region’s stability."
The bombing increased fears that the Syrian civil war would escalate into a regional conflict.
The killed officer was head of the Information Branch of Lebanon's Internal Security Forces, one of the country's two state intelligence services and considered to be aligned against the Assad regime and its Iranian-backed Hezbollah allies.
Hassan's murder, which took place near the headquarters of the Christian Phalange party, threatens to reignite the sectarian tensions behind Lebanon's 1975-1990 civil war and which continue to plague Lebanon.
Sources in Beirut have linked the bombing to Syria and Hezbollah, saying the bombing site was chosen "because it is in the heart of the capital and it is a Christian neighborhood."
Hassan was one of the central figures behind the arrest of former Lebanese Information Minister Michel Samaha on charges of colluding with Assad regime officials to conduct terror attacks and incite sectarian strife inside Lebanon.
He was also at the forefront of the investigation into the murder of former Lebanese Prime Minister Rafik al-Hariri, who was killed by a car bomb in February 2005.
The terror attack ignited the Cedar Revolution that forced Syrian troops to withdraw from Lebanese territory and Hassan later presented evidence to a U.N. tribunal investigating the attack which implicated Syrian-directed Hezbollah agents.
Sectarian tensions in Lebanon had already risen sharply in recent weeks in reaction to Hezbollah's increasingly open cross-border military support for Syrian President Assad.
Backed by Iran, Hezbollah militarily controls southern Lebanon and politically dominates the country.
Its support for Assad, perceived to be at the behest of the Syrian leader's Iranian allies, has undermined the analysis of some Western specialists to the effect that Hezbollah had an indigenous Lebanese movement pursuing Lebanese interests.