Protesters smeared the Russian embassy wall with blood-like ketchup and politicians on Sunday vowed consequences after the Czech government alleged Russian secret services had orchestrated a fatal explosion in 2014.

Citing an intelligence report, Prague said on Saturday evening it would expel 18 Russian diplomats believed to be spies, a step Russia has threatened to respond to with retaliatory measures.

Prime Minister Andrej Babis said Russia's military secret service GRU was behind two blasts at ammunition warehouses in the east of the EU member country in 2014, including one that claimed two lives.

Police then said they were looking for two Russians using passports also held by suspects in the poisoning of former Russian double agent Sergei Skripal in Britain in 2018.

"Killing the citizens of the Czech Republic on its territory by another country, that’s almost an act of war,” protester Tomas Peszynski told AFP at a rally outside the Russian embassy in Prague on Sunday afternoon.

Around 100 protesters sported banners saying "We’re not Russia’s backyard” and chanted "Shame” while waving EU and NATO flags.

The night before, police detained seven people who had smeared the embassy wall with ketchup symbolising blood.

Czech politicians joined the outcry, with government ministers saying Russian companies should not take part in the planned construction of a new nuclear unit worth billions of euros for security reasons.

"I can’t imagine [Russian energy giant] Rosatom getting as far as the security assessment,” Industry Minister Karel Havlicek said.

The two Russians allegedly behind the explosion were identified as Alexander Mishkin and Anatoly Chepigov based on video footage from the site and photos published after the Skripal attack, Czech media said.

Foreign and Interior Minister Jan Hamacek said the second explosion should probably have occurred elsewhere as the men targeted an arms supply to Bulgaria.

Babis said the government would discuss the intelligence report at a meeting on Monday and make it public.

Moscow slammed the expulsion of the diplomats, calling it "a provocation” and threatening retaliation.

In contrast, EU and NATO partners stood by Czech officials, with British Foreign Secretary Dominic Raab saying Britain "stands in full support of our Czech allies”.

"I resolutely condemn the subversive activities targeting the security of... our closest neighbour and ally,” Slovak Foreign Minister Ivan Korcok said in a tweet.