By Nidal al-Mughrabi
GAZA (Reuters) - Israeli troops clashed with Hamas fighters as they advanced into Gaza on Saturday in the first ground combat of an eight-day offensive on the Palestinian enclave, witnesses and the Israeli army said.
Columns of tanks backed by helicopters crossed the boundary fence from four directions into the northern Gaza Strip under darkness, a Palestinian witness said.
An Israeli military spokeswoman said the aim was to seize areas from where Hamas was launching rocket attacks on southern Israel.
"The objective is to destroy the Hamas terror infrastructure in the area of operations," Major Avital Leibovitch said.
The witness said fighting had broken out in northern Gaza as Hamas fighters took on the Israeli forces. Israeli television showed soldiers in battle gear advancing on foot.
Israeli troops and tanks had been massed on the border for days in readiness for an invasion as Israeli firepower pounded Gaza from land, sea and air and diplomatic efforts to arrange a ceasfire stalled.
Israeli officials had repeatedly warned they were prepared to step up military action if Hamas rocket attacks on southern Israel did not stop, but Hamas kept up the action on Saturday.
Large numbers of forces were involved in the operation, including infantry, tanks, engineers, artillery and intelligence, the military said in a statement.
The ground offensive could take many days, the chief military spokesman said.
"This won't be a school outing," Brigadier Avi Benayahu told Israeli television. "We are taking about many long days."
A senior Hamas official said the militants had killed a number of Israeli soldiers but there was no word from Israel on any casualties.
"The Zionist enemy should know that he has four choices of he enters Gaza, first he may be killed, or taken captive, or suffer a permanent disability or return home with a psychological illness," Hamas said in radio broadcast.
Hours before the advance, an Israeli air strike killed 11 Palestinian worshippers, including children, and wounded dozens at a mosque in Beit Lahiya, Hamas officials and medics said.
Rescuers pulled civilian victims from the debris and the bodies lay in pools of blood, witnesses said.
Israel has targeted mosques previously, saying that Hamas had used them as command posts and fire bases.
The mosque raid brought the Palestinian death toll to at least 446, with about 2,050 wounded, in the worst sustained bloodshed in decades of the conflict between Israel and the Palestinians.
Four Israelis have also been killed in the cross-border rocket attacks by Hamas and other militant groups.
Israeli air strikes targeted Gaza from early morning on Saturday and naval vessels also shelled the area from the Mediterranean, witnesses said. One strike killed Abu Zakaria al-Jamal, a senior commander of Hamas's armed wing, Hamas said.
Israel launched the campaign, called Operation Cast Lead, on December 27 saying it wanted to stop the rocket attacks and bring security to its south.
But about 30 Hamas rockets smashed into Israel on Saturday, the military said. Two people were hurt by shrapnel when a rocket hit a building in the port city of Ashdod.
Hamas vowed not to bow to Israel's will.
The attacks brought a wave of international protests and thousands of demonstrators marched in solidarity with the Palestinians in European cities on Saturday.
In Paris, more than 20,000 demonstrators, many wearing Arab keffiyeh headscarves, chanted slogans like "Israel murderer!."
In London, 10,000 protesters led by singer Annie Lennox carried Palestinian flags and placards with slogans such as "End the siege on Gaza" and "Stop the massacre."
NO SLEEP AT NIGHT
The plight of the 1.5 million Palestinians crammed into Gaza was growing more desperate even before the ground attack. People had sheltered in their homes for days and humanitarian agencies warned that food, water and medical supplies were running short.
The electric power plant has shut down and the sanitation system cannot treat the sewage. In the winter cold, fuel for heating and cooking was no longer available, aid agencies said.
"We do not sleep at all at night. We stayed awake the whole night because of the planes," said Umm Kamel, a mother of 11 baking bread on a wood fire in her home in Gaza.
U.S. President George W. Bush said Hamas -- which the United States, Israel's main backer, deems a terrorist organisation -- must take the first step towards a cease-fire.
"Another one-way cease-fire that leads to rocket attacks on Israel is not acceptable," Bush said in his weekly radio address.
Israel occupied Gaza in the 1967 Middle East War and after Palestinian uprisings formally ended its military rule in 2005, although it still controls the borders.
International peace efforts aimed at creating an independent Palestinian state foundered after Hamas won elections in 2006 and drove Fatah forces loyal to Palestinian President Mahmoud Abbas from Gaza a year later.
Hamas called off a six-month truce with Israel last month and stepped up the rocket attacks, complaining at Israeli raids into Gaza and a continuing blockade of the enclave.