Lamy said he would stress the need to resist protectionist tendencies when he meets leaders at the summit.
The build-up to the summit has been marred by increasingly lurid reports about Italian Prime Minister Silvio Berlusconi's private life, who dismissed a stream of newspaper allegations as "all lies" in an eve-of-summit press conference.
"They are all lies, from the underage girls onward," Berlusconi told a news conference when he was asked whether the scandals that have been dogging him since April would cloud the summit.
The meeting begins with a working luncheon at 1:00 pm (1100 GMT) in a heavily fortified police training college on the outskirts of the devastated town.
With aftershocks still being felt, and only days after a tremor hit 4.1 on the Richter Scale, officials have drawn up plans to evacuate the leaders and cancel the summit in the event of a similar quake.
G8 talks traditionally bring together the eight leading economies -- Britain, Canada, France, Germany, Italy, Japan, Russia and the United States -- but much of the discussion will be expanded to include emerging powers such as China and India.
Police made around 40 arrests as anti-globalisation demonstrators hurled bottles at riot squad officers and set fire to tyres in central Rome.
"We will be attentive and we will be careful to ensure freedom of expression, but if someone is thinking of bringing sticks or ballbearings to demonstrations, we will do everything to prevent them," said Interior Minister Roberto Maroni.
Riots marred the last Italian G8 summit, in Genoa in 2001, when a protester was shot dead by police.
The talks will also focus on emerging political crises in China's Xinjiang region, Iran and Honduras.
German Chancellor Angela Merkel said Tuesday she would speak to China's Hu Jintao about the worsening ethnic violence, in which at least 156 people have been killed, on the summit's margins.
Western leaders have plans to push the post-election crisis in Iran to the top of the agenda despite the reservations of China and Russia.
"The Iranian authorities are trying to blame 'foreign powers' for their violent and abusive campaign against peaceful protesters," said Kenneth Roth, Human Rights Watch's executive director. "The G8 needs to make it clear to Tehran that it cannot shift the blame."
The bulk of the summit is likely to focus on efforts to shore up the global economy since London hosted a G20 summit in April where world leaders committed one trillion dollars to help struggling economies and revive global trade.
In their own mini summit on Monday, British Prime Minister Gordon Brown and French President Nicolas Sarkozy said they would soon put forward proposals for talks to address the volatility of the oil market, and press their G8 partners on progressing towards a climate pact.
UN chief Ban Ki-moon plans to press the G8 to live up to previous pledges on climate change and aid for Africa, telling journalists "the time for good rhetoric and half measures is over".
Berlusconi has told reporters that Obama will contribute up to a third of an overall 12 billion dollar pledge for food security in the developing world.
Berlusconi moved the summit to L'Aquila from the fashionable island of La Maddalena in Sardinia to throw international focus on the plight of some 70,000 quake survivors, many living in tented camps.
Before the summit begins, the Italian premier will escort Merkel to the village of Onna, which bore the brunt of the disaster. German money is being used to rebuild the church.
US President Barack Obama, due to arrive Wednesday at the end of a trip to Moscow, and his Russian counterpart Dmitry Medvedev are expected to make similar visits.
The leaders will be accompanied to Italy by their spouses, including French First Lady Carla Bruni-Sarkozy, who will meet quake survivors on Friday.