The two groups have so far refused to voice opposition to the war and their leaders pondered which step is more appropriate.
Peace Now Secretary-General, Yariv Oppenheimer, said Thursday that in light of the cabinet's decision to expand the military operation in Lebanon his group will participate in Thursday's protest.
Peace Now's decision to join the protest is a symbolic as the movement refuses to identify itself with radical left-wing groups.
Peace Now supported the decision to fight Hizbullah citing broad support for the war among the Israeli public and the fact that Israel "had no choice."
"Instead of sending the Israel Defense Forces deep into Lebanon, Olmert has to send (Foreign Minister Tzipi) Livni to the Security Council to seal a political arrangement," Oppenheimer said.
"The cabinet's decision proves that the government has lost direction and entered deep inside Lebanon. I, who at first supported the operation, think the government is not doing enough to bring the war to an end, and is embarking on a dangerous and unnecessary adventure," he added.
Meretz to join
Meretz also declared that it will be joining the protest outside the Ministry of Defense. Former MK Yael Dayan and writer David Grossman will attend the protest.
Radical left-wing parties held a number of small protests against the war in the early stages of Israel's offensive against Hizbullah.