The attack on Wednesday briefly disrupted a rally of thousands of Israeli protesters opposed to this summer's planned withdrawal from the Gaza Strip.
The rally was a show of support for the settlers being uprooted from an area that Israel has occupied for 38 years.
Gaza settler leaders initially said they expected 100,000 people to turn out for the daylong event. However, by midafternoon, police said only about 20,000 people had arrived.
A spokeswoman for the settlers said after the rocket attack that the event would go on.
Early on Wednesday, Israeli authorities closed the main crossing into the Gush Katif bloc of settlements to private cars, allowing only buses through.
The army also closed a main Palestinian road and took up positions on the roof of a nearby Palestinian factory to protect settlers from Palestinian resistance groups.
In a separate development, the army said it arrested two Palestinian teenagers as they tried to cross a West Bank checkpoint with a homemade gun and explosives.