Italian officials said contracts signed in Rome late yesterday would be worth up to 17 billion euros (USD 18.4 billion), topped by a five-billion-euro deal for pipeline company Saipem, whose shares surged 18.5 per cent in Milan yesterday.
A major order for Airbus planes is expected to be confirmed in France tomorrow along with tie-ups with French carmakers Peugeot and Renault.
Rouhani said he had come to Europe with an 'open for business' message in the aftermath of Tehran's nuclear deal with the West.
Italian Prime Minister Matteo Renzi added: "We have signed the first accords but we are only at the start of a long road."
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Renzi said he had discussed efforts to end the war in Syria and the fight against the Islamic State group with the Iranian leader. "If we could reach agreement on the nuclear issue, we can find one on Syria. We can and we have to."
Rouhani is due at the Vatican today before flying to France the next day on his first official European trip as president.
The Iranian leader is accompanied by more than 100 ministers, officials and businessmen.
Rouhani, a 67-year-old former academic and diplomat who is seen as a pragmatist, was elected in 2013 on a pledge to end sanctions and improve relations with the West.
"We have had friendly relations with Italy and France in the past and we want to continue our good relations with them," Rouhani told reporters before his departure yesterday from Mehrabad Airport.