The Met Police said officers had been deployed to a “static protest” on Whitehall, opposite Downing Street on Monday evening.
A picture published by the force showed a huge mass of protesters gathering at the site, carrying Palestinian flags and protest banners.
Separate footage posted on social media showed protesters chanting “ceasefire now”.
The southbound carriageway from Horse Guards Avenue to Parliament Square has been closed due to the protest, the Met said.
“This is necessary to ensure the safety of those present and any passing pedestrians,” they added.
“We will reopen the carriageway as soon as it is safe to do so.”
Protests against Israel’s military operation in Gaza have been taking place regularly in London since the conflict began on October 7, when Hamas fighters killed more than 1,200 Israelis. More than 28,100 Palestinians have died since the beginning of the offensive, according to the Hamas-run health ministry.
The protest comes as Israeli Prime Minister Benjamin Netanyahu faces growing international pressure over plans to invade the southern Gaza city of Rafah, where more than one million Palestinians are believed to be sheltering.
Saudi Arabia on Sunday warned of “very serious repercussions” if Rafah was attacked, while Foreign Secretary David Cameron said that he was “deeply concerned” about the prospect of a military offensive.
Israel claims that Rafah is the last remaining stronghold for Hamas fighters after more than four months of conflict triggered by the October 7 attack.
But aid agencies have warned that an assault on Rafah would be catastrophic. A military operation in the city could force the closure of its crossing, cutting off the delivery of food and medical supplies and exacerbating the already dire humanitarian situation in the besieged territory.
Hamas has said it won't release any more hostages unless Israel ends its offensive and withdraws from Gaza. It has also demanded the release of hundreds of Palestinian prisoners, including senior militants serving life sentences.