Many in the crowd on Sunday blamed the UN for not doing enough to stop Israel's assault on Lebanon and as the numbers grew some protesters broke through police lines and smashed windows in the building.
Tony Maqlini, one of the protesters outside the building, said: "What Israel has done is an act of terrorism. The UN is doing nothing to stop this. They don't care about us."
As the protesters grew increasingly incensed, parliament speaker, Nabih Berri, appeared on television and appealed for calm.
Hezbollah officials also moved through the crowd to stop the protest from becoming a full-scale riot.
For many, Qana is a historic symbol of Israel's heavy-handed tactics in Lebanon. The strike was less than a kilometre away from the mass grave of more than 100 Lebanese civilians who were killed in 1996 after Israel shelled a UN base where they had sought shelter.
Many in the demonstration on Sunday believed that Israel's attack on civilians was not a mistake.
One man, Karim Harmoushe, said: "Israel was saying that there was a rocket launcher in the house but then why didn't any of the people in there notice this and leave?"
The fighting this month between Hezbollah and Israel has cost Lebanon dearly. More than 600 people have been killed, the country has suffered heavy damage to its infrastructure and as much as 25% of the population has been displaced.
Yet despite the heavy losses, the demonstration had a strong air of defiance.
"It doesn't matter how long they attack us for. They should know that we are Lebanese and we know how to fight"
Hani Mansour, protester
One protester, Hani Mansour, said: "It doesn't matter how long they attack us for. They should know that we are Lebanese and we know how to fight and we know how to stand for democracy.
"Mr Bush should know that all the cluster bombs and phosphorous weapons that he is sending to be used against us may cut us to pieces, but each piece will get up again and fight."
The angry mood also extended to Lebanon's leadership, who told Condoleezza Rice, the US secretary of state, that she was not welcome to visit the country.
As Lebanese become increasingly angered by Israel's tactics, Hezbollah appears to be growing in popularity.
A recent poll by the Beirut Centre for Research recorded that 87% of Lebanese from all sects supported the "resistance's fight against Israeli aggression on Lebanon".
Amal Saad-Ghoreyeb, the author of a book about Hezbollah, said, "If the people weren't outraged enough, then all they needed was a surgical precision strike with American-made bombs on civilians. This has definitely radicalised all Lebanese."