In a major escalation of allegations against the current Kremlin government, Mr Johnson told BBC's The Andrew Marr Show: "We actually have evidence within the last 10 years that Russian Federation has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling Novichok".
He then led the enthusiastic crowd to chant "Russia!".
Putin, who is now set to extend his rule until at least 2024 and is already Russia's longest-serving leader since Joseph Stalin, appeared to rule out remaining president for life.
Observers reported widespread ballot-stuffing and unprecedented pressure on Russians to vote, but that is unlikely to seriously damage Mr Putin given his popularity and his tight control over Russian politics.
Mr Putin's main challenges in the vote were to obtain a huge margin of victory in order to claim an indisputable mandate.
With about two thirds of the ballots counted, more than 75 percent were for Putin, according to the Central Election Commission.
The outcome in Sunday's vote is so certain that authorities are investing in get-out-the-vote efforts to ensure a decent turnout.
First-time voters in Moscow were given free tickets for pop concerts and health authorities were offering free cancer screenings.
There had been no doubt that Putin would win in his fourth electoral contest; he faced seven minor candidates and his most prominent foe was blocked from the ballot.
Mr Navalny and his supporters had called for an election boycott but the extent of its success could not immediately be gauged.
Crimea and Russia's subsequent support of separatists in eastern Ukraine led to an array of United States and European sanctions that, along with falling oil prices, damaged the Russian economy and slashed the ruble's value by half.
Britain and Russian Federation last week announced tit-for-tat diplomat expulsions over the spy case and the United States issued new sanctions.
But the disputes likely worked in Putin's favor, reinforcing the official stance that the West is infected with "Russophobia" and determined to undermine both Putin and traditional Russian values.
Most of the voters AFP spoke to said they had backed Putin despite Russia's problems of poverty and poor healthcare, praising his foreign policies.
The 65-year-old former KGB officer used an otherwise lacklustre presidential campaign to emphasise Russia's role as a major world power, boasting of its "invincible" new nuclear weapons in a pre-election speech.
A Ukrainian fighter pilot who was temporarily blamed by Russian Federation for the 2014 downing of Malaysia Airlines Flight MH17 over eastern Ukraine has committed suicide in southern Ukraine.
In his next six years in office, he is likely to assert Russia's power overseas even more strongly.
The Central Election Commission said on Monday morning that it had not registered any serious complaints of violations.
The military campaign in Syria is clearly aimed at strengthening Russia's foothold in the Middle East and Russian Federation eagerly eyes possible reconciliation on the Korean Peninsula as a lucrative economic opportunity.
Russian nationals are electing the country's head of state among eight contenders: Sergei Baburin from the All-People's Union party; Communist Party candidate Pavel Grudinin; incumbent Russian President Vladimir Putin; Civil Initiative party candidate Ksenia Sobchak; Communists of Russia party chairman Maxim Suraykin; Presidential Commissioner for Entrepreneurs' Rights Boris Titov; co-founder of the Yabloko party Grigory Yavlinsky, and head of the Liberal Democratic Party of Russia Vladimir Zhirinovsky.
Russians queue to cast their votes at the Russian Embassy in Berlin, Germany.