By Adama Diarra and Souleymane Ag Anara

BAMAKO/GAO (Reuters) - Malians burned ballot boxes and one candidate was kidnapped during local elections meant to fill posts left vacant in the north since Islamist militants hijacked a 2012 Tuareg rebellion and ousted the government.

The jihadists were driven out a year later by a French-led military operation, but have continued to launch strikes on army and U.N. targets from their desert bases and have intensified their insurgency in recent months and spread further south.

Polls were canceled in at least seven districts for security reasons in elections widely criticized by opposition parties as well as armed groups participating in a U.N.-led peace process, pointing to the ongoing fragility of the former French colony three years after the war.

While locals formed orderly lines outside polling booths in the southern capital Bamako, ballot boxes were burned by armed men in Timbuktu and the PRVM-FASAKO party said its candidate for a commune near the central town of Mopti had been kidnapped.

On a main road near the outskirts of the northern town of Gao, locals had gathered stones and arranged them to read "No elections here", although voting proceeded inside the town and crowds gathered at some polling stations.

"The current situation is not right for elections because the majority of our population are scattered in different refugee camps," said Amgar Ag Yehia, a Timbuktu resident who boycotted the vote.

In Kidal, 250 kilometers to the north in a region that Tuareg separatists call "Azawad", Tuareg women draped in colorful cloth marched in the streets for the second day.

"No elections in Azawad before the appointment of intermediary authorities," said one of their signs, referring to a timeline they say was previously agreed as part of the U.N.-led peace process.

"We have already delayed these elections four times so that they can be inclusive and four times is enough," President Ibrahim Boubacar Keita told reporters after voting on Sunday. Mali's constitution bars further delays.

The opposition Union for the Republic and Democracy party denounced what it called fraud in the vote preparations that it said would benefit Keita's government. A government spokesman was not immediately available to comment on the allegations.

United Nations Secretary General Ban Ki-moon issued a statement late on Saturday asking the Malian government to "pursue a constructive dialogue with all stakeholders to defuse tensions that may arise before and after the poll".

The United Nations has deployed a 13,000-strong peacekeeping mission to help stabilize Mali but it is often the target of attacks and has reported more than 100 fatalities.

(Reporting by Souleymane Ag Anara in Gao and Adama Diarra in Bamako; Writing by Emma Farge; Editing by Tom Heneghan and Louise Heavens)