The number of foreigners overstaying their visa increased this year due to the "Expo effect," local police said.

Police based at Pudong International Airport said 2,159 foreigners have been caught overstaying, a 20 percent increase compared with last year, since the World Expo started on May 1.

Among them, 462 were warned, fined or detained, police said.

According to the country's entry and exit laws, those overstaying their visa can be fined up to 5,000 yuan (US$752) or be detained for 10 days.

Police said many cases involve overseas Chinese who simply forgot the visa duration while they visited their homeland.

Some planned two-week trips were extended to one month, police said.

In other cases, some foreigners confused the visa's validity period for the length of stay, police said.