NOUMEA, New Caledonia (AP) _ Separatist militants on this French Pacific island clashed with police Sunday as voting began in France's presidential election. Two policemen were reported shot in separate incidents.

Meanwhile, Melanesian separatists holding 27 police officers hostage evaded a police search in this French territory and demanded that elections be canceled.

Minister of Overseas Territories Bernard Pons was dispatched from Paris Saturday by Premier Jacques Chirac to oversee the voting.

After the residents of the tiny French islands of Wallis and Futuna, the New Caledonians were the first to cast ballots on Sunday. It was still early morning in France. Voters in the French territories also participate in France's national elections.

Police in Noumea said separatists erected barricades in several villages to disrupt voting. One policeman was reported wounded by a gunshot in Pouembout, on the main island's west coast 160 miles north of Noumea, and a second wounded at Canala on the east coast.

In pre-election violence on Saturday, three security men of a pro-French party were shot and wounded and police fired tear gas in two towns to stop rioting.

The hostages were taken Friday during a battle at a police station that left three officers dead and about 20 people wounded on the tiny island of Ouvea, about 80 miles off the coast of New Caledonia.

Two wounded officers and four others were hospitalized in Noumea, the capital. One officers had been shot in the head and flown to a hospital in Sydney, Australia.

Kanaks, as native Melanesians are known, make up 43 percent of the territory's 145,000 people. The rest are European settlers, Polynesians and Asians.

Many Melanesians seek independence from France, while most of the European settlers wish to remain French. The separatists vowed to boycott Sunday's election and do what they could to interfere with voting.

Leopold Joredie, security minister of the separatist ''government of Kanaky,'' said the kidnappers are demanding that 300 police reinforcements be withdrawn, that Paris send emissaries to discuss independence and that the election be canceled.

Joredie, who has been in contact with the kidnappers, said the hostages were in good condition. However, he added, the kidnappers were ''ready to keep them (the gendarmes) as long as necessary.

''If the government wants to intervene, they must take responsiblity,'' he said.

About 160 additional reinforcements from the National Gendarmerie left Paris Friday to assist the 3,000 police already in New Caledonia.

The kidnappers were believed to have divided into groups and fled into a forest in northern New Caledonia.

A spokesman for the gendarmerie in Noumea, who refused to be identified, said hundreds of officers were searching for the hostages.

Also Saturday, three security men with the pro-French Rally for Caledonia in the Republic party were shot and wounded while driving through Canala.

Jacques Lafleur, the party's president, said the men were in Canala to ''protect those who want to vote Sunday.''

Lafleur's party is the local branch of Chirac's conservative Rally for the Republic.

The Kanak Socialist National Liberation Front, the main separatist organization, is boycotting the elections. It also boycotted last year's referendum on independence, saying only Melanesians should be allowed to vote. The territory voted overwhelmingly for France.

The bloodshed is part of long-simmering tensions in New Caledonia, which has been under direct French rule since 1853 and is France's largest possession in the South Pacific.