Health workers dipped the children's fingers in ink to show they had been vaccinated. In the afternoon, they toured housing complexes and slums in the capital looking for unvaccinated children and offering to give them the vaccine on the spot.

Health officials handed out free balloons and biscuits to the children as they emerged, many of them crying, from clinics in Jakarta and two neighboring provinces.

"I was frightened seeing the paralyzed children on television," said Bambang Sumaryo, who had taken his 15-month-old baby to a health center in south Jakarta. "Rather than that happening, I brought her here today. What is more, it's free."

At Least 16 Cases

Indonesia has detected at least 16 cases of polio since the disease first broke out last month in West Java province, around 74 miles east of Jakarta. The government has already vaccinated children living close to the source of the village where the disease first appeared.

Polio is spread when feces from people with virus come into contact with unvaccinated children, usually via water. It usually attacks the nervous system, causing paralysis, muscular atrophy, deformation and sometimes death. However, only about one in 200 kids infected ever develop symptoms.

Some parents in Indonesia have expressed concerns about possible side-effects of the immunization and the quality of the vaccine, which is produced in Indonesia. The vaccine comes in the form a liquid, a couple of drops of which are squeezed into children's mouths.

"I was concerned my child would get a fever but the doctor convinced me it was safe," said Indarti, a mother in Jakarta. Polio vaccination rates in Indonesia are around 80 percent, but there are pockets where the rate is much lower than that. Until last month, there had not been a case of the disease in Indonesia since 1995.

'Health Centers Are Crowded'

"The people have been very enthusiastic so far today. The health centers are crowded," said Dr. Muhammad Nadirin, head of epidemiological surveillance at the Health Ministry, adding that he expected most the 6.4 million children targeted in the campaign would be vaccinated by the end of Tuesday.

Health workers dipped the children's fingers in ink to show they had been vaccinated. In the afternoon, they toured housing complexes and slums in the capital looking for unvaccinated children and offering to give them the vaccine on the spot, he said.

A second round of vaccinations will take place next month, Nadirin said.

Health officials suspect the latest outbreak was brought into Indonesia by a migrant worker or a Muslim pilgrim returning from the hajj in Saudi Arabia. Polio remains endemic in only six countries -- Nigeria, India, Pakistan, Niger, Afghanistan and Egypt.

Officials say the Indonesian outbreak can likely be traced back to Nigeria, where polio vaccinations were suspended for several months in 2003 after radical Islamic preachers told parents the vaccinations were part of a U.S. plot against Muslims and warned them against having their children immunized.