The president of Equatorial Guinea said a war is being organised against his regime, days after a government official warned of a "destabilisation attempt".

A "war" is being prepared "because they say I have spent a lot of time in power," President Teodoro Obiang Nguema said Saturday, without saying who he was referring to.

On Wednesday neighbouring Cameroon arrested a group of nearly 40 heavily armed men who were headed into Equatorial Guinea.

Equatorial Guinea's Ambassador to France, Miguel Oyono Ndong Mifumu, said the incident was an "invasion and destabilisation attempt".

The country shut its north-eastern border shared with Cameroon and Gabon on Wednesday and has sent reinforcements from the army to the area, sources said.

"The Kye Ossi border between Equatorial Guinea and Cameroon is still closed, even the border with Gabon," residents of the border town Ebibeyin, in Equatorial Guinea, told AFP.

The arrests were followed by the detention of "dozens" of activists on Thursday, according to the main opposition party.

Citizens for Innovation (CI) party said the activists were detained at its headquarters in the capital Malabo and the port city of Bata.

Obiang, Africa's longest-serving leader, called on his supporters to be "vigilant".

"I want a happy transition, I do not want war," the 75-year-old told state television.

"I am not in power because I want to be. When you want to, you can tell me: 'President, you have already worked a lot' and I will go away."

The CI said at least 50 activists had already been detained since a November 12 general election that saw Obiang's ruling party win a landslide victory with 92 percent of the vote.

The election was widely criticised by opposition groups as fraudulent.

Equatorial Guinea, which also borders Gabon, is one of sub-Sahara's biggest oil producers but a large proportion of its 1.2 million population still lives in poverty.