DAKAR (Reuters) - Gambian President Yahya Jammeh warned on Sunday that anyone involved violence or unrest during the country's presidential election in four days time would be dealt with harshly.

"There will be zero tolerance (of) ... violence and anyone disregarding public order will face full force of the law," Jammeh told supporters in Brikama, some 30 kilometers (19 miles) west of the capital Banjul, at a rally broadcast on Gambian TV.

"I am warning all Gambians against violence."

Jammeh and the opposition are campaigning across the tiny former British colony, which straddles a West African river running into the Atlantic whose beaches are a top tourist destination.

Gambian authorities have refused the European Union access to observe the election on Dec. 1, raising doubts about how free and fair it will be, although Jammeh has promised a credible process.

African Union observers have been admitted.

Eight opposition parties are rallied behind businessman Adama Barrow in a bid to end Jammeh's 22-year rule, which activists and diplomats say has been marred by human rights abuses and torture, claims the president's supporters deny.

Nearly 50 protesters were arrested in April and May, including Ousainu Darboe, leader of the opposition UDP party, and at least 18 other senior members. Two have since died during their detention.

(Reporting by Tim Cocks; Editing by Mark Potter)