By Johan Purnomo

CILACAP, Indonesia (Reuters) - Indonesia executed four convicted drug traffickers, three of them Nigerians, in the early hours of Friday, leaving the fate of 10 others uncertain.

The Africans and an Indonesian man were shot by firing squad during a thunderstorm shortly after midnight on Nusakambangan Island in Central Java, as the government ignored international calls for clemency and pushed ahead with what it considers a war on drugs.

The attorney general said on Wednesday that 14 prisoners, including citizens of India, Pakistan, and Zimbabwe, would be executed this weekend.

An official said on Friday the planned executions would go ahead "in stages" but declined to give a timeframe.

Security was stepped up at the Indonesian embassy in Nigerian capital Abuja on Thursday as protesters gathered to urge Indonesia to halt the executions. Indian and Pakistani officials said they were making last-minute efforts to save their citizens.

"We considered several factors and decided that for now four death row inmates would be executed," Noor Rachmad, an official at the attorney general's office, told reporters shortly after Friday's executions.

Just over a year ago, Indonesia executed 14 prisoners, mostly foreign drugs offenders, causing diplomatic outrage.

Rights activists and governments have again called on Indonesia to abolish the death penalty.

But that has gone unheeded by the government of President Joko Widodo, who has said drugs pose as serious a threat as terrorism in what is one of Southeast Asia's biggest markets for narcotics.

The death penalty is widely accepted by the Indonesian public, but police on Thursday had to break up a protest outside the prison by members of a migrant workers group who called for mercy for an Indonesian woman who was scheduled to be executed.

The United Nations and European Union had urged the executions be halted.

"Such death sentences are unlawful and tantamount to an arbitrary execution as they are undertaken in contravention of Indonesia’s international human rights obligations," the U.N. said in a statement on Thursday.

Amnesty International called the executions "a deplorable act that violates international and Indonesian law" and pleaded that the other death sentences not be carried out.

"The injustice already done cannot be reversed, but there is still hope that it won't be compounded," the rights group's regional director, Rafendi Djamin, said.

Around 152 people remain on death row in the country, including convicted drug traffickers from the Philippines, France and Britain. Authorities plan to execute 16 prisoners this year and more than double that number in 2017.

(Additional reporting and writing by Kanupriya Kapoor; Editing by Robin Pomeroy)