The event, at the Gleneagles Hotel near Edinburgh, is the subject of largest security operation in UK history.
Thousands of protestors will stage what is expected to be a peaceful march near the summit venue although some hope to disrupt the running of the event.
An anti-poverty concert will be held in Edinburgh's Murrayfield stadium.
Among those performing include Sir Bob Geldof, U2 and James Brown.
Key issues
The leaders of the eight industrialised nations will start arriving at the Scottish resort in late morning with Prime Minister Tony Blair and President Jacques Chirac both flying in from Singapore.
The Queen will host a dinner for the leaders on Wednesday evening with the official business of the summit beginning the following morning.
Leading up to the summit, pressure has been building on G8 nations to reach deals on debt relief and aid for Africa, address global trade issues and adopt a unified stance in the fight against climate change.
Agreements on 100% debt relief for African countries and a doubling aid for the continent have already been agreed in principle with the leaders having to put the finishing touches to the pronouncements.
However, discussions over trade liberalisation and how to tackle global warming are likely to prove far more contentious.
The United States has played down expectations over trade and environmental issues, President Bush saying any deal to cut farm subsidies would only happen if the EU reformed its Common Agricultural Policy (CAP).
More demonstrations
Up to 5,000 anti-poverty demonstrators are expected to gather in the town of Auchterader, near the summit venue, on Wednesday lunchtime.
A repeat of Monday's violence in the centre of Edinburgh - in which about a thousand demonstrators were arrested - is not expected although police are taking no chances.
Tayside chief constable John Vine said "robust action" would be taken against anyone found breaking the law.
Thousands of people are also expected to gather in Edinburgh on Wednesday to mark the summit's opening ahead of a concert at Murrayfield.
Sir Bob Geldof has called on a million people to congregate in the city to show their support for Africa but estimates have put the likely turnout at close to 250,000.
In advance of the summit, African leaders called on the G8 to "fully embrace" a raft of anti-poverty measures.
Meeting in Libya, African Union heads of state said G8 leaders must act quickly to cancel debt.
During the two-day meeting, G8 leaders are also expected to discuss the global economy, rising oil prices and a range of foreign policy issues including Iraq and the Middle East peace process.
For many, the summit is a defining moment in current world politics, as an upswell of popular support is calling on the G8 leaders to make fundamental changes to the way rich countries deal with poorer nations.
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