The al-Zawahri tape came on the heels of a message from Osama bin Laden, who called for a holy war to liberate the Palestinian territories. Together, the two messages appeared to be a more direct push by the terror network's leadership to use widespread anger over the Gaza violence to whip up support.
Israel's weeklong offensive in Gaza ended in early March. It was launched in an attempt to put down Palestinian militants firing rockets against nearby the Israeli town of Sderot and city of Ashkelon. The Israeli assault killed more than 120 people, including many civilians. Three Israelis also were killed.
Bin Laden and al-Zawahri have frequently referred to the Palestinian cause in their past messages, but usually in broader terms of liberating Jerusalem and denouncing Israeli violence. Their latest calls for attacks, however, had a more immediate and urgent tone.
The string of messages has raised concerns that al Qaeda could be planning new attacks in the West - or is seeking to inspire its sympathizers to carry out violence. In another message last week, bin Laden warned of a "severe" reaction against Europe after Danish papers published a cartoon seen as insulting Islam's Prophet Muhammad.
The authenticity of the 4 minute, 44-second audiotape could not be independently confirmed. But the voice resembled that of al-Zawahri on previous audio and videotapes confirmed to be his. It was posted on an Islamic militant Web site where al Qaeda usually releases its statements, and a banner advertising the tape had the logo of al Qaeda's media arm, Al-Sahab.
"Muslims, today is your day. Strike the interests of the Jews, the Americans, and all those who participated in the attack on Muslims," al-Zawahri said. "Monitor the targets, collect money, prepare the equipment, plan with precision, and then - while relying on God - assault, seeking martyrdom and paradise."
Al-Zawahri said attacks should not be limited to places in Israel and the Palestinian territories.
"Today there is no room for he who says that we should only fight the Jews in Palestine," he said. "Let us strike their interests everywhere, just like they gathered against us from everywhere."
"Let them know that they will get blood for every dollar they spend in the killing of the Muslims, and for every bullet they fire at us, a volcano will turn back on them," he said, referring to American military aid and other ties to Israel. "They cannot expect to support Israel, then live in peace while the Jews are killing our fugitive and besieged people."
Israeli officials did not immediately respond to repeated messages seeking comment.
Al-Zawahri also referred to the publishing of the cartoon seen as insulting Islam's prophet. The cartoons, which sparked deadly riots across the Muslim world in 2006 after they were first published, were reprinted last month.
"They will never be able to insult and make a mockery out of our Prophet, peace and prayers of Allah upon him," al-Zawahri said.
The Egyptian-born Al-Zawahri accused Egyptian President Hosni Mubarak of colluding with Israel in the siege of Gaza. Egypt has sealed its border with the Gaza Strip since the Palestinian militant group Hamas took over the territory last year.
He said Mubarak "repeats the same dirty role" as the Lebanese Phalangists - a Christian militia that was allied with Israel in Lebanon's 1975-1990 civil war and was blamed in the massacre of Palestinian refugees in the Beirut camps of Sabra and Shatila.
"The roles are the same, even if the faces change - the same betrayal even if the names have changed," said al-Zawahri.
Ben Venzke, head of IntelCenter, a U.S. group that monitors al Qaeda message traffic, said in an e-mail that the "direct use of language and sense of urgency" in al-Zawahri's call for attacks - alongside bin Laden's audiotapes last week - "is cause for concern that al-Qaeda may be moving closer to executing a large-scale attack against EU, American and/or Jewish interests."
But Venzke said it's difficult to predict because in the past, attacks sometimes came more than a year after an al Qaeda threat.
In his audiotape posted on the Web on Thursday, bin Laden said, "Palestine cannot be retaken by negotiations and dialogue, but with fire and iron."
There has been little sign in the past of direct involvement by al Qaeda militants in Israeli-Palestinian violence, though Israeli officials and the government of Palestinian President Mahmoud Abbas have warned that the terror network's presence in the area is quietly growing.
With the latest messages, al Qaeda may be responding to impatience among its sympathizers over the lack of direct al Qaeda involvement in the Palestinian issue. Supporters have frequently posted messages on Islamic militant Web forums asking when the group will carry out attacks inside Israel.
The increased focus on the Palestinian issue also reflects the terror network's desire to show it is up to date on the latest events in the region, even as al-Zawahri, bin Laden and other leaders are believed to remain in hiding in the Afghan-Pakistan border region.
The Israeli assault in Gaza was heavily covered in Arab media, particularly Arab satellite TV news networks such as Al-Jazeera, which aired footage of casualties and damage from Israeli strikes in the Mediterranean coastal strip.
The fighting has hampered a new round of peace talks launched at a conference hosted by the U.S. in November. The Bush administration has been pushing the two sides to sign a peace deal by the end of the year.