ST. PETERSBURG, Russia — Faced with meager growth worldwide and a worrisome ebbing of Russia’s own oil and gas revenues, President Vladimir V. Putin announced an ambitious and risky economic stimulus program on Friday along with a novel amnesty plan for imprisoned white-collar criminals that was intended to improve investor confidence.

Mr. Putin’s proposal to dip into the country’s pension reserves for loans of up to $43.5 billion for three big infrastructure projects and other investments provoked an immediate debate among some of Russia’s top financial minds. It also brought warnings from financial experts who said that it might produce a burst of inflation, and that what the Russian economy needed most was deep structural change, to diversify from oil and gas and to build investor confidence.

Mr. Putin, now in his 13th year as Russia’s political leader, made the stimulus plan the centerpiece of his speech at an annual economic forum here that serves as a gathering of the country’s top financial officials, business leaders and foreign investors. Slowing growth has been the obsessive topic this year.

In his speech, Mr. Putin said Russia would distribute at least $14 billion from the reserves as loans to modernize the storied Trans-Siberian Railway, which runs between Moscow and Vladivostok in the Far East; to construct a 500-mile high-speed rail line between Moscow and Kazan, the capital of the Tatarstan region; and to build a superhighway ringing Moscow.