Israeli Army Radio confirmed eight deaths with the number of injured ranging from eight to about 40.
One of the alleged shooters died at the scene, The Jerusalem Post reported. A second assailant reportedly was wearing an explosives belt. Tactical forces were searching the facility for the second gunman.
One student told Israel's Channel 2 that a gunman went into the seminary's library and fired for several minutes with an assault weapon. The student said he shot the gunman twice in the head.
Another student told Haaretz the attack lasted 10 minutes and 500-600 shots were fired.
Police said all students at the Merkaz HaRav Yeshiva have been evacuated. Security forces are deployed in the surrounding Kiryat Moshe neighborhood.
Fifty ambulances responded to the scene, which emergency responders called "a major incident," reported the Arutz Sheva.
Horns honked and people cheered in Hamas-controlled Gaza when residents there heard of the attack, CNN reported.
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