Jeremy Newmark, chairman of the Jewish Labour Movement, said he had spoken to Mrs Cox about projects which aimed to improve work between the Jewish community and international aid and development NGOs.
"She was a very human politician in every sense. She cared. She cared about issues and people," he said.
Further tributes came from communal organisations including the Board of Deputies and Jewish Leadership Council. Chief Rabbi Ephraim Mirvis said Mrs Cox's death had left people "united in grief".
Yuli Edelstein, the speaker of the Knesset, said Israeli parliamentarians felt "deep shock and outrage".
In a letter to John Bercow, House of Commons Speaker, Mr Edelstein wrote that the attack had "crossed a red line".
"Such an incident must set off deafening alarms bells in any society that cherishes life, respects law and order, and abides by basic laws that grant people their human dignity and freedoms," he said, adding that Israel would "stand by you at your dire hour".
Rabbi Herschel Gluck, a campaigner who works with Muslim groups and communities, said he was "devastated".
"I had the privilege of meeting her and speaking on a platform with her. A decent and caring human being, she devoted her life to combating hatred and helping all people," he wrote on Facebook.
As noted by Labour leader Jeremy Corbyn in his tribute in the Commons on Monday, Mrs Cox was a leading advocate for Palestinian rights. In May last year, shortly after taking her seat in the Commons, she described her pride after her first parliamentary question related to humanitarian issues in Gaza.
Hanan Ashrawi of the PLO's executive committee, said Mrs Cox was a "passionate and courageous leader and human rights defender who tirelessly advocated for various causes, including the plight of Syrian refugees, Palestinian child prisoners and an end to the illegal blockade on Gaza".
The Labour Friends of Israel group said it was heartbroken by Mrs Cox's death.
In an article Mrs Cox had written for the Labour Friends of Palestine and the Middle East group about "unlocking Gaza's potential" she suggested extending a ceasefire and "kickstarting" the Palestinian economy.
She had written that Labour should "pledge to support a Palestinian national unity government, with Hamas and Fatah ministers, operating in both Gaza and the West Bank".
Mrs Cox added that her party needed to also "press for an end to rocket or mortar attacks into Israel. Press for accountability for human rights abuses and violations of international law committed by all sides".
