Thousands of people marched through central London to parliament on Saturday calling for a permanent ceasefire in the Gaza-Israel war.
Protesters waved Palestinian flags and chanted: "End the genocide" and "Free, free Palestine".
The demonstration, organised by the Palestine Solidarity Campaign (PSC), also passed a pro-Israel counter-protest that featured portraits of some of the hostages seized in Hamas's October 7 attack on southern Israel that triggered the war.
With a UK general election due to be held on July 4, the group called on people to show political parties that "they must act" by ceasing to arm Israel and demanding a permanent ceasefire "if they want our vote".
The protest came as Israel said its forces had rescued four of the hostages alive from a Gaza refugee camp.
Israel's retaliatory military offensive has killed at least 36,801 people in Gaza, also mostly civilians, according to the Hamas-run territory's health ministry.
"It's so clearly morally wrong what Israel is doing to the Palestinian people," said one demonstrator, 35-year-old Canadian musician Thomas Rapsey.
"We want the bombing to stop," he said.
Another protester, who asked to be identified only as Lucy, called Israel's actions in Gaza "horrific".
"We've seen a genocide on our TV now for eight months, and no one in the world seems to care or do anything about it.
"It's been ignored by the world leaders and major institutions so it's important for me to feel that I could do something," the 26-year-old research analyst told AFP.
They support a ceasefire, if both sides in the conflict agree, and the release of Israeli hostages. They also back the creation of a Palestinian state as part of the Middle East peace process.
A member of Starmer's top team said recently that the UK should pause arms sales to Israel to stop an offensive in the southern Gaza city of Rafah -- a position the British government has rejected.
The October 7 Hamas attack on Israel resulted in the deaths of 1,194 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Some 251 people were seized by the militants. There are now 116 hostages remaining in Gaza, including 41 the army says are dead.