By Nidal al-Mughrabi
GAZA (Reuters) - Israel launched a ground offensive in the Gaza Strip on Saturday, sending tanks and infantry into battle with Hamas fighters who have defied eight days of deadly air strikes with salvoes of rocket fire into Israeli towns.
Israeli tank fire killed three Gazans at the onset of the night-time invasion, bringing the Palestinian death toll since December 27 to nearly 450, medical officials said. Israel said the operation in the Hamas-run enclave could take "many long days."
Hamas said it killed several soldiers, but there was no Israeli confirmation as columns of tanks pushed into the coastal territory from four entry points.
Heavy casualties are likely to increase international pressure on Israel to halt its biggest operation in the Gaza Strip in four decades, fighting that holds significant political risks for Israeli leaders ahead of a February 10 national election.
The plight of the 1.5 million Palestinians crammed into Gaza was growing more desperate. People have taken shelter in their homes for days and humanitarian agencies warned that food, water and medical supplies were running short.
Several hours into the armoured offensive, Israeli tanks moved some two kilometres (1.2 miles) into the northern Gaza Strip, taking up positions in an area frequently used by militants to fire rockets across the border, witnesses said.
A Palestinian petrol station along the invasion route was engulfed in flames after being hit by a tank shell.
A spokesman for Hamas's armed wing, the Izz el-Deen al-Qassam Brigades, said Israeli troops faced certain death or capture.
"The Zionist enemy must know his battle in Gaza is a losing one," said the spokesman, Abu Ubaida.
At least a quarter of the 449 Palestinians killed in the current conflict have been civilians, a U.N. agency said. Another 2,050 Palestinians have been wounded.
Four Israelis have been killed by rockets that continue to pound southern Israel, where residents were told to stay home and take cover in rooms made of reinforced concrete as the ground forces moved across the frontier.
Israeli Defence Minister Ehud Barak, in a television address shortly after the ground push began, said its aim was to "protect the home front" from rocket attacks. He stopped short of any threat to try to topple the Hamas government in the enclave.
"It won't be easy. It won't be short. I don't want to delude anyone," said Barak, leader of the centre-left Labour party, and a candidate for prime minister in an election that opinion polls predict will return right-winger Benjamin Netanyahu to power.
Large numbers of forces were involved in the sweep, including infantry, tanks, engineers, artillery and intelligence, the military said in a statement.
"The objective is to destroy the Hamas terror infrastructure in the area of operations," Major Avital Leibovich said.
Israeli troops face Hamas militants whom the United States and Israel say have received arms and training from Iran. Hamas is believed to have about 25,000 fighters and has placed landmines and other traps in anticipation of an invasion.
An Egyptian-brokered six-month truce expired on December 19 but it had been strained by Hamas rocket strikes and Israeli military operations against the group.
The United States said before the ground offensive started that it was working for a durable truce but insisted Hamas stop firing rockets first.
Former British Prime Minister Tony Blair, representing major powers sponsoring Middle East peace talks, planned to begin shuttling on Sunday between Israeli leaders in Jerusalem and Palestinian leaders in the occupied West Bank.
But divisions within the European Union over the Israeli operation could reduce pressure on Israel to cease fire. Blair does not plan to meet Hamas, which opposes the statehood negotiations.
On Monday, French President Nicolas Sarkozy is scheduled to go to Jerusalem. Israel had rejected Paris's calls for a 48-hour cease-fire to allow more humanitarian aid into the Gaza Strip.
But diplomats said the Europeans were drafting a new proposal for a truce. Political sources said some of Israel's leaders favour a formal cease-fire backed by the U.N. and major powers. Other decision-makers in Israel say it would only tie their hands should rocket fire continue.
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.
One strike killed Abu Zakaria al-Jamal, a senior commander of Hamas's armed wing, Hamas said.
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."
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-and-a-half later.
(Writing by Jeffrey Heller and Adam Entous; Additional reporting by Dan Williams; Editing by Dominic Evans)