Shadow Welsh secretary Jo Stevens's Cardiff office was daubed in red paint and covered in posters which accused her of having "blood" on her hands.
She called the incident "intimidating" and "threatening".
Labour leader Sir Keir Starmer's stance on the Israel-Gaza war caused amajor rebellionwith 56 of his MPs voting for an immediate ceasefire.
Beth Winter was the only Welsh Labour MP to back the ceasefire.
The red paint and banners appeared on Ms Stevens's office in Albany Road, Roath, on Thursday night, with some accusing the Cardiff Central MP of supporting the deaths of babies in Gaza.
Another poster said the Labour MP had "blood on your hands".
Ms Stevens toldBBC Radio Wales Breakfastshe was shocked by the damage, which occurred during an event she said was described as a "vigil".
"I absolutely support the right to protest, but what was done last night has gone way beyond that," she said.
"If you have someone write murder across your door, it is intimidating."
Adam Johannes from the Cardiff Central Anti War group said about 200 people gathered outside the office.
"We don't feel our MP is representing the majority opinion in our constituency," he said.
"We had a march down Albany Road, returned back and dispersed."
Mr Johannes said organisers asked people to  protest "peacefully and respectfully", but added: "I think we have to look at the bigger picture. After the vote... many people said they felt a sense of powerlessness.
"We're seeing very catastrophic realities, emotions are running high."
He said the group would like Ms Stevens to arrange a forum in which the situation in Gaza could be openly debated with constituents.
South Wales Police said it was investigating criminal damage and that "a number of items have been seized for examination".
First Minister Mark Drakeford hasechoed the stancetaken by Sir Keir, who called for longer "humanitarian pauses" to allow aid to reach civilians in Gaza.
The issue has divided Labour, with ten of Ms Stevens's fellow frontbenchers stepping down from their jobs over the vote, including eight shadow ministers.
Ms Stevens said she "found it particularly surprising" that a former Member of Senedd had been present at the protest.
"I would have thought at the very least, that this individual would have had some understanding of the effect that this will have on my team," she added.
"This is a workplace and my staff team and I, as well as my constituents who come to my office every day for help, should be able to do so in safety."
Former Plaid Cymru MS Bethan Sayed said she had taken part in the protest "peacefully", and accused Ms Stevens of failing to "hear and respect" the views of her constituents who support a ceasefire.
"I took part peacefully," Ms Sayed said in a statement. "I'd urge Jo Stevens MP to consider her actions as opposed to focusing on me."
Plaid Cymru's three MPsvoted for the ceasefirebut their parliamentary leader, Liz Saville Roberts, criticised the damage caused to the office.
"Jo and I voted differently this week and I strongly disagree with Labour's stance," she said.
"But attempts to intimidate elected representatives through vandalism and harassment are unacceptable and counterproductive."
Israel launched a major military campaign in the Gaza Strip to destroy Hamas, which it considers a terrorist group, in retaliation for the 7 October cross-border attack by hundreds of gunmen.
At least 1,200 people were killed in Hamas's assault on Israel and about 240 others were taken hostage.
Since Israel started its counter-attack, Gaza's Hamas-run health ministry has said 11,500 people have been killed in the territory and the United Nations has warned of a "humanitarian disaster".
Police have launched an investigation after a Labour MP who abstained on the Gaza vote had her office vandalised.