Lula said before departing for Tehran that he was "optimistic" about the visit, and that he hoped to persuade Ahmadinejad to reach an agreement with the West over its nuclear work.
"I must now use everything I have learned over my long political career to convince my friend Ahmadinejad to come to an agreement with the international community," Lula said.
Turkey's prime minister, Recep Tayyip Erdogan, has postponed a trip to Azerbaijan and will fly to Tehran on Sunday night to join in the nuclear negotiations, according to Turkish government officials.
But the US and Russia have already said the chances of success are weak.
Brazil and Turkey, both non-permanent members of the UN Security Council, have so far resisted US-led efforts to push through a fourth set of sanctions against Iran over its failure to accept repeated ultimatums to stop enrichment activity.
Iran has rejected a UN proposal to enrich abroad the uranium it says it needs for a nuclear research reactor.
The West fears Iran wants highly enriched uranium to make an atomic weapon, a charge Tehran vehemently denies.
"We have received many proposals and we are considering them," Ali Akbar Salehi, Iran's atomic chief, was quoted as saying on Saturday in local media.
"There is a willingness on both sides to resolve the problem and things are moving positively."
Manouchehr Mottaki, the foreign minister, indicated that Iran was still not ready to budge from its position.
"We hope that the parties [involved in the talks with Iran] will bend to the realities and choose the right path," he was quoted as saying in reports on Saturday.
Dmitry Medvedev, the Russian president, said on Friday that his Brazilian counterpart's talks with Iran "may be the last chance before the adoption of appropriate decisions within the framework of the Security Council".
That came after a senior US state department official said on condition of anonymity on Thursday that the Lula visit was being seen as "perhaps the last big shot at engagement"
Lula has in the past defended Iran's nuclear activities, saying Tehran has the right to atomic energy, and has repeatedly said sanctions would be counter-productive and ineffective.