Commonwealth Secretary-General Don McKinnon has said that the 53-nation club should pull together to fight for a fair global trade deal.
The discussions are taking place in an informal retreat
Mr McKinnon, addressing a twice-yearly meeting of the Commonwealth nations, said that trade was the "most potent weapon" to combat poverty.
He said World Trade Organisation (WTO) talks, which resume in Hong Kong next month, needed a "shot in the arm".
Terrorism and human rights are also expected to dominate the summit.
Addressing the opening ceremony, Mr McKinnon said: "We joined together in the struggle against apartheid at the end of the last century. Now it is the scandal of poverty that demands our attention.
"If we can give the WTO the shot in the arm it requires, then our organisation will have shown again its global worth," he said.
Commonwealth leaders represent about 1.8 billion people, or 30% of the world's population. Member countries, which are mainly former British colonies, include wealthy Canada and Australia and poor Sierra Leone and Malawi.
Queen Elizabeth II, addressing the three-day meeting as head of the Commonwealth, said the group's diversity was its strength.
"Determined and collective action can also help us tackle other challenges that cannot be addressed alone, such as the scourge of terrorism, which is a threat to us all and has directly affected a number of our countries," she said.
Commonwealth leaders, who will spend two days of the summit behind closed doors at an exclusive hotel, are also expected to discuss human rights.
Mr McKinnon said he would raise the subject with Uganda's president, amid concerns over elections there.
The Ugandan opposition leader, Dr Kissa Besigye, who recently returned from exile to fight presidential elections in March, has been charged with treason, terrorism, illegal possession of weapons and rape. He is due to appear in both a military and civilian court on Friday.
There could be frank exchanges between President Yoweri Museveni and other leaders alarmed at the timing of the charges, says the BBC's Mike Wooldridge in Malta.
The subject of trade reform will be the "one issue that leaders will be very united on", Mr McKinnon said.
He said that most leaders "are getting really angry at the lack of ambition" surrounding the WTO trade negotiations in Hong Kong next month, aimed at forging a new global trade deal and lifting millions of people out of poverty.
"There are too many people in either Geneva or Brussels saying that it's time a lot of developing countries lowered their expectations. I don't think they intend to lower their expectations," Mr McKinnon added.
Developing countries at the summit are also expected to lobby UK Prime Minister Tony Blair to press the European Union to slash farm subsidies at the WTO talks in Hong Kong. Britain currently holds the presidency of the EU.