DAKAR, Senegal (AP) — Three Gambian opposition party members have died in detention following a demonstration earlier this week calling for electoral reforms in the tiny West African nation, the United Nations said Sunday.

U.N. Secretary-General Ban Ki-moon in a statement Sunday said he was dismayed by the death of United Democratic Party member Solo Sandeng and two fellow party members. He called for an independent investigation into the deaths in state custody, and called for the immediate and unconditional release of others arrested.

"The Secretary-General is deeply concerned about the apparent use of excessive force and the arrest and detention of peaceful demonstrators," the statement said.

On Thursday, Gambian youths and activists demonstrated for electoral reforms in Serrekunda, 7 kilometers (4 miles) from Banjul, Gambia's capital. Security personnel opened fire and arrested 25 demonstrators, the opposition party said.

Opposition leader Ousainou Darboe was arrested Saturday with other party supporters who gathered in Serrekunda calling for the release of those in custody, including those who died in detention, the party said.

The party called on the international community to intervene to stop "the brutal crackdown on the opposition," saying it only wants a free, fair and credible electoral process.

"Gambian leader Yahya Jammeh has put in place mechanisms to not only entrench himself in power by constricting the democratic space but also using all illegal means and the full force of the security forces to visit terror on defenseless Gambians," it said.

Gambia is to hold presidential elections in December.

Rights groups accuse President Yahya Jammeh, who has ruled since 1994, of human rights violations.

The U.S. State Department in a statement Sunday condemned the Gambian government's response to the protests.

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AP writer Abdoulie John in Dakar, Senegal contributed to this report.