Hackers took down the Israeli fire and rescue service website overnight Thursday, posting messages supporting armed resistance against Israel, Israeli media reported.
Israeli TV Channel 2 said the saboteurs identified themselves as the Gaza Hackers Team, and warned of more cyber attacks.
Israel has vowed to hit back at computer hackers after thousands of Israelis' credit card details were posted online last week, by a hacker who said he was based in Saudi Arabia.
"We will take firm action against those who compromise our security including through cyber-terrorism, and if necessary we will use international law enforcement ... Cyber-terrorism is the new battleground and just as we defeated our opponents on every other field ... we will defeat this as well," Deputy Foreign Minister Danny Ayalon said.
The attack on the fire service ridiculed his statement, posting a picture of the minister over-set with footprints.
"Why do you not understand that this is an embarrassing situation for you and that you cannot get out of it? Danny Ayalon said that Israel will respond to hacking of Israeli sites. We break into your sites every day and will continue to do so without interruption," the hackers wrote.
They also posted: "Hamas, Fatah, Islamic Jihad and all the Palestinian factions will fight your army. All prisoners will be released from your prisons. Freedom for Palestine."
The website was functioning normally on Friday morning.
Israel on Thursday called on computer hackers not to take the law into their own hands to avenge attacks on Israeli credit card companies, and said the authorities were capable of countering all cyber threats.
After last week's publication of thousands of personal and credit card details, at least one Israeli hacker declared he had carried out a reprisal cyber-attack on Saudi credit card holders, although the scope of his action could not be verified.
"We call on Israeli citizens to abide by (the law). Just as the Israeli government has found answers for terrorism, we will find answers to this challenge ... we call on Israeli citizens not to ... act as vigilantes," Deputy Foreign Minister Danny Ayalon said in a statement.
The hacker, identifying himself as Saudi-based OxOmar, said last week he had leaked private information about more than 400,000 Israelis. Credit card companies said around 25,000 numbers, some of them expired, had been posted.
After Israeli media ran what they described as interviews conducted with OxOmar by email, the Haaretz newspaper said a blogger had tracked the hacker down and determined he was a 19-year-old citizen of the United Arab Emirates studying and working in Mexico.
Hamas described OxOmar's actions as "a new form of resistance."
"We urge Arab youth to ignore these cowardly Israeli threats and to use all means available in the virtual space to confront Israeli crimes," Hamas spokesman Sami Abu Zuhri said in Gaza on Sunday.