Seven people, including three children, have been killed by Israeli shells which hit a beach in the northern Gaza Strip, Palestinian officials say.
The shelling has caused outrage among Palestinians
At least 30 people were wounded in the shelling, they say.
The Israeli military says it has halted all shelling of Gaza and has launched an inquiry into whether ground-based artillery could have been involved.
In a statement, the military wing of Hamas threatened to resume attacks on Israel in the wake of "massacres".
The group has been observing a self-imposed ceasefire for more than a year.
Although there have been threats of a response to other attacks in recent months, the BBC's Simon Wilson in Jerusalem says the move is significant because it appears on the official website of the armed wing of the group.
There was no immediate word from the political wing of Hamas, which dominates the government in the Palestinian Authority.
Four other people were also killed in separate Israeli air strike in northern Gaza on Friday, Palestinians said.
The incidents come a day after senior Palestinian official Jamal Abu Samhadana was killed in an Israeli air strike in Rafah, the southern Gaza.
Samhadana - the founder of the Popular Resistance Committees (PRC) - was buried in Rafah on Friday, with thousands of mourners pledging to avenge his death.
Samhadana was one of Israel's most wanted men in Gaza, and his group has been blamed for a series of missile attacks on Israel.
Palestinian leader Mahmoud Abbas condemned the Israeli strikes in Gaza.
"What the Israeli occupation forces are doing in the Gaza Strip constitutes a war of extermination and bloody massacres against our people," Mr Abbas said in a statement carried by the Palestinian official Wafa news agency.
UK Foreign Secretary Margaret Beckett said London was "deeply concerned by reports of the deaths from Israeli shelling of civilians, including children, on a Gaza beach".
Palestinian officials say the seven people killed on the Gaza Strip beach included two women as well as the three children.
The first television pictures revealed a terrible scene, the BBC's Alan Johnston says.
TV pictures suggest a family was on a picnic when the disaster struck
At least four figures lay unconscious on the ground, possibly dead, our correspondent says.
A little further away, a man was lying on a sand dune, perhaps fatally injured, while a child stood looking on in utter horror, our correspondent says.
He says around the casualties were tables and chairs, and it looks very much as if this was a family enjoying their Friday afternoon off on the beach when disaster struck.
An Israeli army spokesman said Chief of Staff Dan Halutz had ordered an immediate stop to all artillery shelling of Gaza while an investigation was carried out into the beach shelling.
The spokesman told the BBC initial findings suggested the shell which killed the people could not have been fired from an Israeli naval vessel or from the air.
He said the army was investigating the possibility that it may have been fired by ground-based artillery.
For many months, the Israelis have been pounding away at open areas such as fields and orchards in an effort to prevent Palestinian militants using them to fire their home-made missiles into nearby Israeli territory.