The marchers, waving yellow Hezbollah flags and Lebanese and Palestinian flags, said the Palestinians and Lebanese are united in their fight against Israel.
Israel launched a major offensive into southern Lebanon on Wednesday after Hezbollah killed eight Israeli soldiers and captured two more. The Israeli-Hezbollah fighting erupted just three weeks after Israeli forces entered Gaza to win the release of a soldier captured by Palestinian militants and to halt rocket fire from Gaza into Israel.
At Gaza City's parliament building, Hamas protesters burned Israeli and American flags, and vowed to strike inside Israel.
"We in Hamas came here to tell our people in Lebanon your blood is our blood, your enemy is ours and your aim is ours," a Hamas protester shouted into a loudspeaker.
Palestinian Prime Minister Ismail Haniyeh of Hamas told reporters he is "really worried about Lebanon" and called for logic and reason to prevail.