Lavrov joind the session, chaired by EU diplomacy chief Federica Mogherini, shortly after his arrival to Lausanne, Switzerland.
The plenary session is being held with the participation of the Iranian delegation at the talks.
Earlier on Tuesday, Lavrov said that chances were "high" that the latest round of negotiations in Lausanne would be successful.
The talks were scheduled to conclude last Sunday but were prolonged till Tuesday as the sides look to work out a framework agreement ensuring the peaceful nature of Iran's nuclear program by the end of March.
Initially, Tehran insisted on having at least 10,000 centrifuges, but in February agreed to consider a US proposal of cutting the number to 6,500.
Iran and the P5+1 group have held several rounds of talks on Tehran's nuclear program since November 2013 with limited results. The negotiators are now working towards the July 1 deadline to forge a final agreement.