Officials said it showed Iran's nuclear plans were on track despite sanctions aimed at forcing it to curb uranium enrichment which many countries fear is aimed at developing atomic bombs.
But the head of the United Nations' nuclear watchdog called on Iran to address concerns about its true intentions and several European energy companies said they were reducing their dealings with the Islamic Republic due to sanctions.
"This day will be remembered ... because it was the day when fuel was lowered into the core of the reactor," said Ali Akbar Salehi, head of Iran's Atomic Energy Organisation.
Amid great media fanfare, fuel rods were transported into the reactor building in August, but they were not inserted into its core and the plant's start-up was delayed due to what were described as minor technical problems.
At a much lower-key news conference, broadcast live from the plant on Iran's coast, Salehi said it would take a further two months to complete the process of lowering 163 fuel assemblies into the core of the reactor and running tests. He said three fuel assemblies had been inserted so far.
The Russian-built 1,000-MW plant will feed Iran's first nuclear power into the national grid early next year, he said.
"If it were in Europe it would supply electricity to about 800,000 or 900,000 people," said Ian Hore-Lacy, of the World Nuclear Association (WNA) industry body.
Iran has denied that the "Stuxnet" computer virus delayed the start-up, although it did infect some computers at Bushehr. Some analysts speculated the worm was designed by Iran's enemies to sabotage the nuclear programme.
TALKS STILL STALLED
"Iran's peaceful nuclear activities are going on as scheduled," Foreign Ministry spokesman Ramin Mehmanparast told reporters at his weekly news conference.
Alaeddin Boroujerdi, the head of Iran's parliamentary committee on foreign policy and national security, called the fuelling of Bushehr a victory against sanctions.
"What counts a lot in this process is that America mobilised all its resources across the world to ratchet up the pressure on the Islamic Republic of Iran and they believe that imposing sanctions on us will deter us from making progress," he said.
A U.N. Security Council resolution passed in June, imposing a fourth round of sanctions, renewed a call on Iran to suspend uranium enrichment, something Tehran has explicitly refused to do, saying such activity is its right under international law.