Protests blocking deliveries of some of the UK’s major newspapers have been condemned as “completely unacceptable” by the Prime Minister.

Some newsagents’ shelves were left empty on Saturday morning after Extinction Rebellion (XR) protesters targeted Newsprinters printing works at Broxbourne, Hertfordshire, and Knowsley, near Liverpool overnight.

More than 100 demonstrators used vehicles and bamboo lock-ons to block roads, with both protests concluding by Saturday afternoon.

Merseyside Police said it had arrested 30 people, while Hertfordshire Police said it had taken 50 people into custody.

Writing on Twitter, Boris Johnson said: “A free press is vital in holding the government and other powerful institutions to account on issues critical for the future of our country, including the fight against climate change.

“It is completely unacceptable to seek to limit the public’s access to news in this way.”

The Federation of Independent Retailers (NFRN) said the protests had hit home delivery operations, including for the “elderly and vulnerable”, with its members having to deal with “angry customers”.

Newsprinters presses publish the Rupert Murdoch-owned News Corp’s titles including The Sun, The Times, The Sun On Sunday and The Sunday Times, as well as The Daily Telegraph and Sunday Telegraph, the Daily Mail and Mail On Sunday, and the London Evening Standard.

XR apologised to newsagents for the disruption but added it would not apologise to Mr Murdoch, calling on him to “stop suppressing the truth about the climate crisis and profiting from the division your papers create”.

Responding to criticism from Home Secretary Priti Patel that its actions were an “attack on our free press”, XR said: “Our free press, society and democracy is under attack – from a failing government that lies to us consistently, is becoming increasingly authoritarian, and is leading us towards 4 degrees of warming.”

Culture Secretary Oliver Dowden condemned XR’s actions as “idiotic”.

He tweeted: “We must never take our free press for granted, and those who disrupt our access to it damage our democracy.”

Jo Stevens, Labour’s shadow digital, culture, media and sports secretary, said: “A free press is vital for our democracy. People have the right to read the newspapers they want.

“Stopping them from being distributed and printers from doing their jobs is wrong.”

But in a now-deleted tweet, Labour MP Dawn Butler appeared to praise XR, writing: “Bravo #ExtinctionRebellion. Excellent work…”

Newsprinters condemned the protests as an “attack on all of the free press” which had affected workers going about their jobs, and others such as newsagents who face a “financial penalty”.

The company said it had transferred printing to other sites, but that delays would occur in some deliveries of The Sun, The Times, the Daily Mail, the Daily Telegraph and the Financial Times.

Ian Murray, executive director of the Society of Editors, said: “Everyone has the right to peacefully protest and make their voices heard, after all that is what a free press is all about. But it is not acceptable for those who wish only their voices to be heard to attempt to silence others.”

Police said no arrests were made after XR protesters held a demonstration near Motherwell aimed at disrupting the distribution of Saturday’s Scottish Sun newspaper.

There was a large police presence in central London on Saturday afternoon as XR staged other events.

Officers were seen carrying and leading away protesters from a peaceful “Citizens Assemble!” gathering in Trafalgar Square who refused to move on after sitting on the ground and listening to speeches.

A five-a-side football match was staged underneath Nelson’s Column to highlight criticism of Barclays.

Protesters claimed the bank, a partner and former title sponsor of the Premier League, is the “largest funder of fossil fuels in Europe”.

Meanwhile, XR said a 20ft model boat named after teenage activist Greta Thunberg was being dismantled and taken away by police in Kennington, south London.

The Lightship Greta, featuring the words “Sound the alarm – Climate Emergency”, had been marched on foot from Brighton.