Prosecutors in Spain Tuesday called for the head of Rwanda's intelligence service Karenzi Karake to be handed over to Spanish authorities after his arrest in London, a judicial source said.

British police detained the 54-year-old at Heathrow airport on Saturday on a Spanish arrest warrant issued in an investigation of alleged crimes during the Rwanda conflict in the 1990s.

British police have arrested the head of Rwanda's intelligence service on a warrant issued by Spain, officials said Tuesday, sparking outrage from Kigali.

The police said Karenzi Karake was held on suspicion of war crimes, apparently in reference to a 2008 arrest warrant from a Spanish court for genocide, war crimes, crimes against humanity and terrorism.

Rwanda's ruling party have backed calls to change the constitution that would allow strongman Paul Kagame a third term in power as president, reports said Tuesday.

Over 3.7 million people have signed a petition calling for a change of Article 101 of the constitution, which limits the president to two terms, the New Times newspaper reported.

The list of supporters is long: more than two-fifths of Rwanda's voters have signed a petition calling for constitutional reform to allow strongman Paul Kagame a third term in power.

The authorities speak of a popular mass movement, critics of a manipulation of power.

Burundi's opposition parties have threatened to boycott elections later this month if the government fails to create the conditions for a fair vote, a spokesman for a coalition of 17 parties said Friday.

The troubled central African nation has been in crisis since late April over President Pierre Nkurunziza's controversial bid to stand for a third consecutive five-year term, a move branded by opponents as unconstitutional and a violation of a 2006 peace deal that ended 13 years of civil war.

Rwanda's Green Party, the country's tiny but main opposition, said Thursday it was challenging moves to change the constitution to allow strongman and President Paul Kagame to stand for a third consecutive term in elections in 2017.

"We submitted a lawsuit to the Supreme Court yesterday, demanding the court block parliament from any future plans to reform the constitution, especially article 101 about the lifting of presidential term limit," party president Frank Habineza told AFP.

The United States on Sunday helped evacuate American, Canadian and other foreign citizens from Burundi, rocked by weeks of deadly street protests and an attempted coup, a State Department spokesman said. 

Burundi's streets have been mired in violence since the announcement by President Pierre Nkurunziza that he would stand for a third term in office. 

U.N. troops are preparing an offensive against Hutu rebels in the Democratic Republic of Congo after plans for a joint operation with the Congolese army fell through, the U.N. peacekeeping chief said Thursday.

Herve Ladsous told reporters following a closed-door meeting of the U.N. Security Council that the U.N. mission MONUSCO would carry out unilateral operations to drive out the Hutu militia based in eastern DR Congo.

Protesters in Burundi clashed with police for a third day Tuesday in fresh demonstrations against the president's bid to cling to power for a third term, witnesses said.

At least five people have died since clashes broke out Sunday after the ruling CNDD-FDD party, which has been accused of intimidating opponents, designated President Pierre Nkurunziza its candidate in the June 26 presidential election.

Burundi's president was Saturday declared as candidate for a controversial third term in office, his ruling party announced, despite mounting protests over a move that the opposition says is unconstitutional.

There were fears the political crisis, during which President Pierre Nkurunziza's ruling CNDD-FDD party has also been accused of intimidating opponents, could push Burundi back into violence.