A crowd at a pro-Israel rally booed Sen. Ed Markey on Monday after suggesting violence should be de-escalated in the Middle East nation's war against Hamas.

Hamas "are violent extremists," Markey, a Massachusetts Democrat, told the crowd. "They gain support when there is a crisis."

"That is why the United States and the international community must keep pushing for diplomacy and the ending of civilian casualties on all sides," Markey continued. "There must be a de-escalation of the current violence."

Boos erupted, and the lawmaker briefly paused his speech. 

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Hamas terrorists launched a surprise attack on Israel using sea, ground and air troops in tandem with a barrage of thousands of rockets, killing hundreds and wounding thousands. Israel declared war against the Gaza-based group Saturday night in response and began a retaliatory assault in Gaza.

As of Monday afternoon, nearly 1,600 people have been killed, including around 900 in Israel.

Senator Ed Markey

Sen. Markey, a Democrat, was booed on Monday at a pro-Israel rally in Boston. (Eric Lee/Bloomberg via Getty Images)

Rallies supporting either side have cropped up throughout the U.S. in the days following, including in New York, San Francisco, and Washington, D.C.

Rep. Jake Auchincloss, a fellow Massachusetts Democrat who also spoke at the Boston rally, rebuked Markey's call for de-escalation.

ISRAEL RECAPTURES AREAS NEAR THE GAZA STRIP OVERRUN BY HAMAS

Airstrike hits Ashkelon, Israel

Israelis evacuate a site struck by a rocket fired from the Gaza Strip, in Ashkelon, southern Israel, on Monday, Oct. 9. (AP/Ohad Zwigenberg)

"De-escalation is not possible when they are taking hostages," Auchincloss told the crowd. "And Israel did not ask America to de-escalate on September 12, 2001."

"Hamas is an internationally recognized terrorist organization that is executing and raping civilians," he continued. "Israel is a liberal democracy with the right and responsibility to defend itself and its citizens."

Israel needs moral support from Americans and the U.S. should not be ambiguous in their support for their Middle Eastern ally, Auchincloss added.

Hamas, as part of its attack, seized hostages, including women, children and older civilians. The terrorists said they would broadcast an execution of a hostage every time an Israeli airstrike hit a Gaza target without forewarning.

Netanyahu speaking at podium

Israeli Prime Minister Benjamin Netanyahu said Israel will finish the war that Hamas started. (SEBASTIAN SCHEINER/POOL/AFP via Getty Images)

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Israeli Prime Minister Benjamin Netanyahu addressed his country on Monday amid his military's fight against Hamas.

"Israel is at war," Netanyahu said. "We didn’t want this war."  

"It was forced upon us in the most brutal and savage way," he said. "But though Israel didn’t start this war, Israel will finish it."