By Ralph Ellis

CNN

(CNN) -- Between 75 and 100 tombstones were overturned and damaged Saturday night at a Jewish cemetery in Philadelphia, police said Sunday.

The incident at the Mt. Carmel Cemetery in the Wissinoming neighborhood is being classified as institutional vandalism, but investigators have not established a motive, said Detective Jim McReynolds of the police department's Northeast Detectives Division.

Cemeteries are located on each corner of the intersection, but no vandalism was found at the three Christian cemeteries, he said.

"As far as we know it's limited to the Jewish cemetery," he said.

The estimate of 75-100 damaged tombstones was conservative and may grow, he said.

Rabbi Shawn Zevit of Mishkan Shalom Synagogue said he and a member of his congregation went to the cemetery and counted "a few hundred" headstones knocked over.

While not providing an exact number, he said "the sheer scale is quite something."

'A holy site'

The vandalism was especially worrisome because it comes less than a week after a similar incident at a Jewish cemetery in St. Louis, said Nancy Baron-Baer, the Anti-Defamation League's regional director for the Eastern Pennsylvania, South Jersey and Delaware region.

"One stone in one cemetery being desecrated is one stone too many. We are talking about hundreds within a week," she said.

The ADL is offering a $10,000 reward for information leading to the arrest and conviction of those responsible, she said.

The Fraternal Order of Police Lodge No. 5 offered a $3,000 reward for of those responsible.

Naomi Adler, CEO of the Jewish Federation of Philadelphia, said the Mt. Carmel cemetery dated to around 1890. Once the damage is assessed, the federation will organize a cleanup effort, she added.

"Maybe we'll never know if this was an anti-Semitic act, but it's a desecration of a holy site," Adler said.

'Very disheartening'

The vandalism was discovered Sunday morning by a man visiting his father's grave, CNN affiliate WPIV reported.

"It's just very disheartening that such a thing would take place," Aaron Mallin of New Jersey said.

Philadelphia police said they received a call at 9:45 a.m. Sunday about the vandalism.

"Upon their arrival at the listed location, responding officers were met by the complainant, who reported that (3) three headstones belonging to his relatives were damaged, due to being knocked over," police said in an email. "The cemetery was inspected and approximately 100 additional headstones were found to be knocked over; it appears that the incident occurred sometime after dark on Saturday, Feb. 25, 2017."

"I'm a child of a Holocaust survivor, so I grew up with stories of destruction of Jewish cemeteries," Rebecka Hess told WPIV. "I always thought we were done with that."

She was part of the hundreds of community members who came to the cemetery on Sunday to help fix the vandalized headstones.

'Cowardly, disturbing act'

Social media users responded with outrage.

The Anne Frank Center for the United States urged President Trump to take action, tweeting: "Jewish graves in Philadelphia now vandalized. Stop this incubator of #Antisemitism and other hate. WE BEG YOU @POTUS @realDonaldTrump"

Pennsylvania Gov. Tom Wolf tweeted: "The vandalism of Jewish headstones at a Phila. cemetery is a cowardly, disturbing act. We must find those responsible and hold accountable."

Philadelphia Mayor Jim Kenney issued a statement urging solidarity. "Hate is not permissible in Philadelphia," he said. "I encourage Philadelphians to stand with our Jewish brothers and sisters and to show them that we are the City of Brotherly Love and Sisterly Affection."

A local rabbi, Yosef Goldman went on Facebook to say community support was immediate, writing: "Members of the local Ahmadiyya Muslim Community have showed up to help us right the headstones. They are calling their youth to come join us. #sacredresistance"

Bomb threats

Last Monday, it was discovered that more than 100 headstones had been toppled at the Chesed Shel Emeth Society cemetery in the St. Louis suburb of University City, police said.

Vice President Mike Pence and Missouri Gov. Eric Greitens visited the cemetery and condemned the vandalism.

Muslim American activists Linda Sarsour and Tarek El-Messidi launched a fund-raising effort to help pay for the toppled headstones to be repaired.

Bomb threats have also recently plagued Jewish groups.

From the start of the year through Monday, 69 bomb threats have been made to 54 Jewish centers in the United States and Canada, according to the JCC Association of North America.

No bombs were found, and no one was injured in connection with the threats, according to the organization, which is working with law enforcement and the FBI to investigate the calls.

CNN's Evan Simko-Bednarski, Kristina Sgueglia, Dalila-Johari Paul, Jason Hanna, Alexandra Larkin and Faith Karimi contributed to this report.