The short session was held to begin the legal procedures needed to officially nominate the candidates for the November 4 presidential election - McCain and Sarah Palin, governor of Alaska and the vice-presidential candidate.
Outside the convention around 10,000 people attended a largely peaceful anti-war march, although police say around a dozen were arrested after authorities used pepper spray on several hundred protesters.
Marchers chanted anti-war slogans and waved signs criticising George Bush, the US president, some carrying signs saying "McSame=Bush".
In opening speeches, Laura Bush, wife of the current US president and Cindy McCain, wife of John McCain, the Republican presidential hopeful, both called on the audience to donate funds to those affected by the hurricane.
"This is a time when we take off our Republican hats and put on our American hat," Cindy McCain said, as a giant screen showed the names of state-approved charities in the affected states of Alabama, Florida, Louisiana, Mississippi and Texas.
George Bush, the US president, and Dick Cheney, his vice-president, have scrapped plans to attend the convention and will be in Texas visiting relief stations there, officials said.
However McCain is still expected to address the convention on Thursday at the end, officials said.
McCain ordered most activities on Monday, the first day of the convention, to be suspended.
He also visited a disaster centre in Waterville, Ohio, during campaigning on Monday.
"We will try not to overly politicise it, but we felt it was important to encourage support into these areas," Rick Davis, McCain's campaign manager, said.
The party has been swift to alter its convention plans in a bid to avoid being seen as holding a political celebration while Hurricane Gustav hits the south of the country.
Al Jazeera's Rob Reynolds in St Paul says both McCain and his Democratic rival, Barack Obama, are trying to appear above the partisan fray by telling Americans to work together to assist those affected by the hurricane.
Bush and Cheney's absence will also assist McCain as their appearance could reinforce in voters' minds the connection between the two men at a time when McCain is making his own bid for the White House, our correspondent adds.
Earlier on Sunday police also arrested nine people taking part in large anti-war protest for tresspassing after they crossed a security fence into a restricted area near the Xcel Energy Centre where the convention is taking place, security officials said.