MOSCOW — Russian opposition leader Alexei Navalny was discharged from a Moscow hospital on Monday and returned to prison under guard after being treated for what his lawyer and doctor have described as suspected poisoning from a chemical agent.

Navalny, 43, was rushed to hospital from jail on Sunday with what his spokeswoman said were signs of an acute allergy with "severe swelling of the face and skin redness."

He was arrested several days before a major opposition rally on Saturday that ended with nearly 1,400 people detained — the largest number of detentions at a rally in the Russian capital this decade, according to a monitoring group.

Police said the protesters numbered about 3,500 but aerial footage from several locations suggested at least 8,000 people turned out.

Alexei Navalny attends a rally in memory of slain politician Boris Nemtsov in Moscow on Feb. 24. Tatyana Makeyeva / Reuters file

The Moscow hospital treating him said on Monday it had discharged him after his condition had improved. It said it could not disclose what it believed had been behind his sudden illness due to patient confidentiality.

Elena Sibikina, one of the doctors who treated Navalny, told reporters that the idea that he had been poisoned with a chemical substance had "not been proven." She said his life was not in danger.

His own doctor and lawyer said they opposed the "strange" decision to return Navalny to a prison cell where they believe he was somehow poisoned in the first place.

"He was really poisoned by some unknown chemical substance," Olga Mikhailova, his lawyer, told reporters. "But what the substance was has not been established."

Navalny's sudden hospitalization infuriated supporters who suspect he may have been targeted as part of one of the biggest crackdowns in recent years against the opposition.

The fierce Kremlin critic and anti-corruption campaigner is currently serving a 30-day sentence for violating tough protest laws after urging people to take part in the Saturday demonstrations.

Dmitry Gudkov, an opposition figure who was barred from running for city council office in Moscow, was detained Sunday afternoon as he delivered food to some of the Moscow protesters still in jail.

The U.S. Embassy in Moscow on Sunday decried the violent crackdown as "use of disproportionate police force" and the Russian presidential human rights council said it was concerned about the police brutality.

Russian President Vladimir Putin stayed away from Moscow over the weekend.

On Sunday, he led Russia's first major naval parade in years, going aboard one of the vessels in the Navy Day parade in St. Petersburg, on the Gulf of Finland. The parade included 43 ships and submarines and 4,000 troops.