US President George Bush has warned North Korea that it will face intense pressure to resume six-party talks on its nuclear programme.
More than 4,000 military-style police were on the streets
"The leader of North Korea will hear a common voice," he said, ahead of the Apec summit in Santiago, Chile.
He said the US, China, Japan, South Korea and Russia all wanted to rid the Korean peninsula of nuclear weapons.
Police and protesters have clashed outside the summit venue, and further demonstrations are expected later.
American officials say Mr Bush is expected to use the Apec summit to map a strategy to resume talks with Pyongyang over its nuclear ambitions.
He held separate meetings with the Chinese, Japanese, Russian and South Korean leaders before the summit opened.
1989: Set up to promote free trade
1993: Bill Clinton proclaims 'Pacific century'
1997: Confidence hit by Asian financial crisis
1999: East Timor crisis
US negotiators have denied reports that some of the countries want more concessions to North Korea, beyond a proposal made in June to allow South Korea and Japan to provide the North with aid.
More than 20 nations are represented at the gathering, where the discussions are expected to focus on trade liberalisation and counter-terrorism.
Thousands of police have been deployed throughout Santiago to break up unofficial protests.
The authorities declared a holiday on Friday but up to 25,000 people marched through Santiago in protest against capitalism and US policy in Iraq and elsewhere.
Apec members account for more than half of global economic output and almost half of all international trade.
The bloc was set up in 1989 mainly to promote trade among the Pacific rim countries.
Apec summits have a habit of being overshadowed by other issues, and this year's looks set to be no different, says the BBC's Elliott Gotkine in Santiago.
Australia, Brunei, Canada, Chile, China, Hong Kong, Indonesia, Japan, South Korea, Malaysia, Mexico, New Zealand, Papua New Guinea, Peru, Philippines, Russia, Singapore, Chinese Taipei, US, Vietnam, Thailand.
Inside the convention hall, the leaders are due to discuss ways to combat terrorism.
There are also hopes that Apec could take steps towards creating a free trade area for the Asia-Pacific region, our correspondent adds.