Cop27: Egyptian MP heckles Sanaa Seif as she campaigns for Alaa Abd el-Fattah's release
Amr Darwish, a pro-government parliamentarian, was captured on video grabbing a microphone and assailing Seif and other speakers in Arabic before security intervened.
"You are here on Egyptian land, don't touch me," he shouted in English at the security guard.
Agnes Callamard, secretary general of Amnesty International, could also be heard in the video shouting "freedom of speech" at Darwish.
Seif has been vocal at the conference on the plight of her brother, an icon of the 2011 uprising who has spent eight of the past 10 years in jail on various charges.
Following a seven-month hunger strike during which he only consumed 100 calories per day, Abd el-Fattah stopped drinking water on Sunday to coincide with the opening of the United Nations Cop 27 climate summit in the Red Sea resort.
Widely considered Egypt's best-known dissident, he has been sentenced to five years in prison for "spreading false news", having already spent the better part of the past decade behind bars.
Seif, her sister Mona, their mother Laila Soueif and her aunt, celebrated novelist Adhaf Soueif, have campaigned worldwide for the release of the activist, who gained British citizenship through his UK-born mother in April.
"We need sensible people to intervene," Seif said in an interview with AFP.
"I put my hopes in the British delegation because, as his sister, I can't give up or tell myself that my brother will die."
She acknowledged the risk of travelling to Sharm el-Sheikh, with its heavy security restrictions, saying: "I admit, I was afraid to come. But it's our last resort.
"I came so that Alaa wouldn't be forgotten. I want to remind both Egyptian and British officials that my presence means that someone is dying and that it's possible to save him."