The attacks came just days after an unidentified hacker, proclaiming Palestinian sympathies, posted the details of thousands of Israeli credit card holders and other personal information on the Internet in a mass theft.

Stock trading and El Al flights operated normally despite the disruption, which occurred as Israeli media reported that pro-Palestinian hackers had threatened at the weekend to shut down the TASE stock exchange and airline Web sites.

While apparently confined to areas causing only limited inconvenience, the attacks have caused particular alarm in a country that depends on high-tech systems for much of its defense against hostile neighbors. Officials insist, however, that they pose no immediate security threat.

"They have demanded an apology for Israel's defensive measures," Deputy Foreign Minister Danny Ayalon said on his Facebook page, alluding to the conflict with Palestinians.

"I am using this platform to send a clear message that ... they will not silence us on the Internet, or in any forum."

The First International Bank of Israel (FIBI) and two subsidiary banks, Massad and Otzar Hahayal, said their marketing sites had been hacked but that sites providing online services to clients were unaffected.

Israel's third-largest bank, Discount, said it had been spared attack, but that it was temporarily shutting down foreign access to its website as a precaution.

The Tel Aviv bourse website could only be accessed intermittently, but screen-based trading was not hit.

"There has been an attack by hackers on the access routes to the website," said Orna Goren, deputy manager of the exchange's marketing and communications unit. "The stock exchange's trading activities are operating normally."

El Al said it had taken precautions to protect the company site and warned of possible disruptions to its online activity.

"CYBER WAR"

There was no claim of responsibility for Monday's incidents.

However, the Islamist group Hamas, which governs the small Palestinian territory of Gaza, welcomed the attacks as a blow against the Jewish state, which it refuses to recognize.

"This is a new field of resistance against the Occupation and we urge Arab youth to develop their methods in electronic warfare in the face of (Israel's) crimes," Hamas spokesman Sami Abu Zuhri said in Gaza.

Israeli Information Minister Yuli Edelstein told a conference in Tel Aviv that the cyber attacks were part of a wider move to smear the country's reputation and "threaten Israel's economic stability and security."

"It's another episode in the war our enemies are conducting as a campaign of delegitimization to hit our pockets and lifestyle," he said, in reported comments confirmed by his spokesman.

"Israel must use all measures at its disposal to prevent these virtual dangers from turning into real threats and to prevent with all its force attacks against it and its institutions. Today it's credit card theft and toppling Web sites, and tomorrow it could be theft of security information and harm to infrastructure."

Israel opened an agency to tackle cyber attacks earlier this month. A founding member of the unit, Isaac Ben-Israel, said the country's most vital systems were already protected, but that incidents like the ones seen recently would only increase.

"As long as the systems are not guarded, any hacker anywhere in the world can break into them and do damage," Ben-Israel said on Israel Radio. "I believe that, done right, in a year or two, we will be able to wipe out all these hackers' threats."

(Additional reporting by Allyn Fisher-Ilan and Steven Scheer in Jerusalem, Tova Cohen in Tel Aviv and Nidal al-Mughrabi in Gaza; Editing by Crispian Balmer and Alastair Macdonald)

• A Permanent Economic Emergency