"This massacre will not pass without punishment," Ismail Haniya, a senior leader of the group in Gaza, told reporters.
"This massacre will only make our people stronger and more determined to continue the resistance."
Hamas had earlier said in a statement that six members of its armed wing, the Izz al-Din al-Qassam Brigades, were killed in al-Nusairat refugee camp while another three were killed in al-Buraij camp.
Among those killed in the operation was a local leader of the Brigades, 44-year-old Hasan Zahut, it added.
Palestinian medical sources said three boys, aged eight, 12 and 15, were among those killed in the clashes around the entrance to the neighbouring camps. More than 80 other people were wounded, they added.
Witnesses to the Sunday raids said occupation soldiers shot dead the young men as they attempted to halt the Israeli military as it forced its way into the camp.
Dozens of armoured military vehicles took part in the sweeps. Tanks choked the main roads and explosions thundered through the camps in the early morning.
The Israeli army said the operation in al-Buraij was carried out "in order to prevent terrorist attacks carried out against Israeli targets".
But Aljazeera's correspondent witnessed Israeli helicopters firing randomly in the camps.
"Huge numbers of Israeli forces, backed by tanks and jeeps, are still surrounding the area, opening fire at citizens' houses," he reported.
Palestinian security sources told Aljazeera the Israelis were using a new type of explosive bullet.
The Hamas website quoted a communique from Izz al-Din al-Qassam saying it had knocked out two Israeli tanks during the invasion, and scored direct hits on two more.
Raiding refugee camps
Israeli commentators had speculated the army would step up operations against Palestinians in the Gaza Strip before Prime Minister Ariel Sharon puts into motion an evolving plan to evacuate some settlements in the area.
According to an Israeli military source, the raid was a "pinpoint operation against the terrorist infrastructure in the central Gaza Strip".
Young Palestinians throw stones
to stop Israelis blocking off camp
The source said Palestinians from the refugee camps in the area had been attacking Israeli convoys and firing mortar bombs and Qassam rockets at Jewish settlements.
The raids came after Palestinian resistance fighters mounted an elaborate but unsuccessful attack on Saturday at the Israel-Gaza Erez border in which fighters used fake Israeli jeeps.
However, Israeli border guards opened fire on the first jeep, killing two men in the lead vehicle before fighters could launch their attack.
The second jeep exploded prematurely – killing its occupants and two policemen uninvolved in the attack.
For thousands of Palestinian labourers, Erez is a gateway to coveted jobs in Israel and in an Israeli-run industrial zone at the crossing.
There were no Israeli casualties in the Erez operation.