Twelve people were arrested as hundreds joined a pro-Palestinian al-Quds Day demonstration on one side of the Thames, while hundreds more gathered on the opposite bank to back Israeli and American attacks on Iran.

Al-Quds Day is an international demonstration of support for Palestinian rights. The event takes its name from the Arabic for Jerusalem and was established by Ayatollah Ruhollah Khomeini after Iran’s 1979 revolution.

At least 1,000 police officers were drafted in to keep the two rival protests apart. Lambeth Bridge, the nearest river crossing to each rally, remained closed on Sunday afternoon.

The Metropolitan police assistant commissioner Ade Adelekan said: “We made 12 arrests including for showing support for a proscribed organisation, affray and for threatening or abusive behaviour. We are also investigating chants made by a speaker at the al-Quds protest.”

The Met later confirmed it was investigating chants of “death, death to the IDF” led by Bobby Vylan, whose real name is Pascal Robinson-Foster. The member of the punk-rap duo Bob Vylan led the same chant at Glastonbury festival last year, prompting police to open an investigation that later concluded with no further action taken.

The home secretary, Shabana Mahmood, granted a police request to ban al-Quds protesters from marching, for the first time since 2012. She said: “I expect to see the full force of the law applied to anyone spreading hatred and division instead of exercising their right to peaceful protest.”

Police warned the al-Quds demonstrators on the Albert Embankment that they would arrest those displaying placards, flags or chanting that “cross the line into hate crime or support of a proscribed organisation”.

Some did carry placards of Iran’s new supreme leader, Mojtaba Khamenei, or his father and predecessor, Ali Khamenei, who was killed on the first day of the war.

There was also a heavy police presence over the Thames at Millbank at a counterprotest co-organised by Stop the Hate and the Lion Guard of Iran group.

Some protesters carried both an Israeli flag and the flag of the Iranian state before the 1979 Islamic revolution to show support for Reza Pahlavi, the son of Iran’s former pro-western monarch.

Georgie Stagg, 70, a retired arts administrator from Lewisham who was wearing a Palestinian keffiyeh, walked past the pro-Israeli demonstration on her way to the al-Quds rally. She was quickly moved on by a police officer, who said: “Because of what you are wearing, unfortunately I’m going to have to ask you to move that way.”

Stagg said: “We’ve marched on al-Quds Day for 40 years, and I have never seen any trouble. There was friction last year, but that’s because the pro-Israel people were down at Parliament Square, along with a lot of far right who were causing the trouble.”

Stagg added: “You can’t criticise the Iranian government for being anti-democratic when we’ve got a government here that’s arresting people holding placards. We were told you couldn’t say ‘from the river to the sea’, and you can’t say global intifada – that just means uprising.”

The rival demonstrations exposed bitter divisions between Iranians in the UK. One of the pro-Israeli protesters carried a banner saying: “Qud you take your terrorism and fuck off.”

Raham Moshami 52, fled Iran in 2010 after being tortured in jail. She showed scars on her forehead as proof. “We are here to support our people, because the Iranian government is holding my people hostage,” she said.

She added: “Netanyahu and Trump are trying to help us. We have to remove a cancer, because the Iranian government is like a cancer. Pahlavi is a good man, he’s very educated.”

Moshami also dismissed the al-Quds demonstrators as being in the pay of the Iranian government, without offering any evidence for the claim.

On the other side of the Thames, Fereydun Bahrami, 71, had travelled by coach from Glasgow with 50 other Iranians to join the al-Quds protest, which is named after the Arabic name for Jerusalem and was organised by the Islamic Human Rights Commission.

“We are here to celebrate al-Quds Day and also protest against the war,” he said. Bahrami, who left Iran to study engineering in Glasgow where he went on to run a metalworks, was carrying a placard that read “Stop Using UK bases to bomb Iran”.

Gesturing across the river he said: “They are brainwashed to support Israel, rather than their families under bombardment.”

He added: “I’m very sad about this war. At the beginning America killed 168 girls in a school. For the last 47 years Iran hasn’t attacked anyone.”

Bahrami objected to government ministers suggesting the demonstration was a hate march. “This is a love walk, how can it be a hate march?” he said. “We love human beings. We love Jewish people. We are Muslims, we are bound by religion to love everybody else. There is no hate in this crowd here.”

Salma, 60, who works for a freight company in London, said she supported Iranian retaliation against targets in the Gulf. She said: “Fighting back and standing up to the Americans is right – Trump had no right to go in and take out an 86-year-old leader.”

She added: “You can’t have one side being allowed to bomb schools and then worry about ships waiting in the strait of Hormuz. If petrol prices are going up, who cares? Because at the end of the day, this was created by the US.”

Adelekan said fewer people attended the march than anticipated due to the restrictions. “Our policing plan worked, with both groups kept apart and we saw no attempts from either side to breach conditions by marching. Both sets dispersed as planned from 1500 hours,” he said.

“The restrictions and conditions meant many people chose to stay away and not to attend the protest or counterprotest.”