Seven Palestinians have been killed by Israeli tank fire in northern Gaza.
The dead were all youngsters, according to their relatives, killed after they rushed outside to watch an exchange between militants and Israeli soldiers.
The Israelis say six of the dead were 17 and older, and that some were members of the militant group Hamas.
Palestinian leader Mahmoud Abbas, in unusually inflammatory language, prayed for the "martyrs who fell today to the shells of the Zionist enemy".
" We are praying for the souls of our martyrs who fell today to the shells of the Zionist enemy "
He was speaking to thousands of supporters at a rally in Khan Younis, a stronghold of Palestinian militant groups, and later cancelled a visit to the hospital that received the casualties amid fears of more violence.
Mr Abbas, viewed by the US and Israel as the best Palestinian partner for peace negotiations, is heavily tipped to become Palestinian president in an election on Sunday.
He opposes the continuation of the armed uprising against Israel.
In a separate incident later on Tuesday, a Palestinian was killed in a shoot-out with Israeli soldiers east of Gaza City, Palestinian hospital officials said.
Mutilated
The seven killed were hit by Israeli fire in a strawberry field near the town of Beit Lahiya, close to Gaza's border with Israel.
The Israeli army says it opened fire on a group of nine masked men equipped with missile launchers, to stop them firing mortars at nearby Israeli targets. An Israeli civilian had reportedly been injured a short while before.
Palestinian witnesses, however, say the militants escaped before the tank opened fire and it was the children and teenagers who were hit as they rushed out of a farmhouse to watch the action.
Dr Mahmoud al-Asli, director of a hospital in Beit Lahiya that received the casualties, said the dead were aged between 11 and 17, the Associated Press reported.
Hospital sources also said that some of the dead were so mutilated that it initially proved difficult to identify them.
Relatives added that six of them were from the same family and that three of the boys were brothers.
Palestinian militants have been waging a four-year intifada or uprising against Israel, whose forces have occupied the Gaza Strip and West Bank since 1967.
Israeli Prime Minister Ariel Sharon is planning to pull out all 8,000 Israeli settlers and the troops who protect them from 21 fortified enclaves in Gaza. Israel will maintain control of Gaza's borders, coastline and airspace.
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