Alexei Navalny
Yulia Navalnaya joined the platform on Monday, three days after her husband's death in a Russian penal colony.
The former House speaker put the ex-president on blast for his fealty to the Russian leader.
The Russian opposition figure died in a Russian penal colony on Friday.
More than 400 people were detained across the country at the weekend while trying to honour the politician's memory.
"He will probably be remembered as the best president of Russia that Russia will never have," the Polish foreign minister said.
Jailed Kremlin Critic Navalny Pokes Fun At Prison Officials With Demands Of Moonshine And A Kangaroo
Imprisoned Russian opposition leader Alexei Navalny has released excerpts of his correspondence with prison administrators.
An estimated 40,000 people took to the streets of Moscow on Saturday, with rallies held across almost 40 cities and towns.
Trump described the situation as "tragic", adding that while he hadn't seen "any proof" of poisoning he would investigate.
Novichok, a Soviet-era nerve agent, was used to poison former Russian spy Sergei Skripal and his daughter in Britain.
“We assume that Alexei was poisoned with something mixed with his tea. That’s the only thing he drank this morning.”