The protests were on the second day of an Israeli ground offensive that saw the death toll in Gaza top 330 people.
In Toronto, demonstrators held signs with slogans like "Gaza needs more than our prayers," chanting "free, free Palestine," across the street from the Israeli Consulate.
Organizer Hind Awwad said the rally was drawing attention to what she calls "Israel's ongoing aggression."
She said the Harper government's "unwavering support" of Israel amounts to complicity in "crimes in the Gaza Strip."
"We're here to raise our voices and say Israel must be pressured," she said, adding that protesters are calling for sanctions.
Aidan Macdonald, who is also an organizer with the Coalition Against Israeli Apartheid, said the goal was to "express solidarity" with Palestinians.
"As long as the assault continues, there are going to be people mobilizing in the streets," he said.
Macdonald added the Canadian government's response to the conflict has been "appalling," with officials refusing to condemn the deaths of Palestinian civilians.
In Montreal, several hundred people marched through the streets in the city's north end to denounce the attacks.
Jaoudat Abouazza, who emigrated from the West Bank in 1977 and still has family throughout the region, said it's been harrowing watching the tragedy unfold.
"We want peace. We need peace," said Abouazza, who came to the protest with his wife and three children.
"It's very upsetting. It's hard to accept what's been happening there."
Tempers briefly flared at the protest after a woman showed up waving a small Israeli flag.
Terri Allister, a Jewish Montrealer, told demonstrators and reporters that Israel "had the right to defend itself."
One man angered by her presence tried to rip the flag away from her, causing her to lose her balance.
Another woman gave her a hug, and said all the protesters want is peace.
There was a similarly tense scene in Calgary on Friday evening, where a group of Israel supporters showed up at a pro-Palestinian rally and a fight broke out.
Dania Jamous, a 23-year-old Montrealer, said the conflict has been going on for far too long and must stop.
"Children being murdered is not acceptable," she said.
"We need to go into the street and denounce it."
Placard carrying protesters marched in Vancouver and urged Israel to stop its military operations in Gaza and were also critical of Prime Minister Stephen Harper.
"Our government has been pro-zionist and we want to stand against that," said Dania Kallas, a protest organizer.
The Israeli military has said it has severely diminished the arsenal of Hamas, the Islamic militant group ruling Gaza, but accuses militants of continuing to fire rockets.
Harper spoke with Israeli Prime Minister Benjamin Netanyahu on Saturday to discuss the situation.
A spokesman for Harper said the two men discussed the need to "make every effort to minimize civilian casualties" and Harper reaffirmed his support for Israel's right to defend itself.
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