Hundreds of anti-austerity protesters took part in impromptu protests across the UK on Saturday, with a London rally starting outside the Conservative Party headquarters.

Protestors threw traffic cones, smoke bombs and other objects at the police in central London, angry that the Conservatives won the general election.

Thousands gathered in the impromptu protests, where Officers in riot gear clashed with a “minority” of protesters who were demonstrating against further austerity cuts to public services. The police described the protest as an “unplanned” demonstration.

A police investigation is also under way after the phrase “Fuck Tory Scum” was graffitied onto the Women’s War Memorial in Whitehall. This comes just as the Queen attended a service of thanksgiving at Westminster Abbey to mark the 70th anniversary of VE Day – the end of World War Two in Europe.

A Downing Street spokesman condemned the graffiti as “a despicable display of disrespect for those who fought and died for their country”.

In a statement, the Met said that 12 people were arrested on suspicion of violent disorder and three for assault on police. Of the 15 arrested, 14 have been bailed pending further enquiries including a full review of CCTV footage and a 24-year-old man remains in custody after being arrested on suspicion of assault on police.

Two officers were treated at hospital after the clash with protestors, while three other officers were assaulted but did not go to hospital. There have been no reported injuries to any protesters.

Chief Superintendent Gerry Campbell said the vast majority of protestors had taken part peacefully, but added that a “small minority” had been intent on causing disorder.

“We have launched an investigation into criminal damage to the Women in World War Two Memorial. This is particularly abhorrent considering we are celebrating the 70th anniversary of VE Day,” he said.

A similar, albeit smaller, rally against austerity took place in Cardiff over the weekend. Roughly 200 people were protesting, including the singer Charlotte Church.

In Bristol on Thursday, thousands also took to the streets in a more peaceful anti-austerity demo. Campaigners turned out in droves to oppose the £12bn welfare cut the Conservative party plans in a bid to eradicate the deficit by 2018.

Although there were no arrests, major traffic delays were caused around the centre of Bristol throughout the evening.

The former leader of the Conservatives on Bristol City Council, Peter Abraham, told the BBC he was disappointed the march was taking place.

“I don’t accept this word austerity,” he said.

“This is the far left and the rag-tag and bobtail of protest groups coming together and just making a fuss.

“Perhaps they are doing it because they don’t like the [election] result but the people spoke and that’s democracy.”

More protests are being planned for the following months, with The People’s Assembly against Austerity hosting events online to encourage protesting in May and June. The group is also thought to be behind the London protests that took place last weekend, although that hasn’t been confirmed.

The People’s Assembly against Austerity have posted an event on facebook for May 27th, calling for a protest starting outside of 10 Downing Street and moving down to the Emmanuel Centre in Westminster during the evening of the Queen’s Speech.

They have also called for a National Demonstration on June 20th, starting outside the Bank of England. The group states: “With the Tories going it alone in government we know exactly what to expect.

“More nasty, destructive cuts to the things ordinary people care about – NHS, the welfare state, education and public services.”