Monday's controversial referendum in the Comoros could allow its president, Azali Assoumani, to seek re-election and retain power beyond 2021, when his currently non-renewable term would otherwise end.Several other African leaders have also sought to hang on to power through changes to the constitution.Here are some examples. Success In Rwanda, voters in 2015 overwhelmingly backed a referendum that removed term limits from the constitution, allowing President Paul Kagame - in power since 1994 - to potentially rule until 2034. The same year the Republic of Congo (Brazzaville) voted by a landslide on constitutional changes that would allow veteran ruler Denis Sassou Nguesso to run for a third term. He was re-elected in 2016. In Zimbabwe a new constitution adopted in 2013 let Robert Mugabe stand in another election, which he won. He was forced to step down in 2017, after 37 years in power. Chad's Idriss Deby Itno has been in power since 1990 thanks to a constitutional revision in 2005 that was adopted after a disputed referendum.In Uganda, also in 2005, a constitutional reform scrapped limits on presidential terms. Yoweri Museveni, in power since 1986, was re-elected to a fifth term in 2016.In Togo in 2002 a constitutional amendment allowing the president to seek reelection without limit paved the way for Gnassingbe Eyadema, in power since 1967, to win another term.When he died in 2005, his son Faure Gnassingbe took over and has since won three contested elections. The opposition is seeking a return to a two-term limit. Crisis Burundi was plunged into a bloody political crisis after Pierre Nkurunziza won a highly controversial third term in 2015, which the opposition said was unconstitutional.In May 2018 Burundians voted overwhelmingly in a referendum in favour of constitutional changes that included extending presidential terms, potentially allowing Nkurunziza to stay in office until 2034. But a month later Nkurunziza announced he would step down in 2020.The Democratic Republic of Congo's President Joseph Kabila was obliged - but refused - to step down at the end of 2016 after he reached his two-term constitutional limit.The Constitutional Court ruled he could remain in office until his successor was elected. After much delay, sparking deadly protests, a vote is scheduled for December 23, 2018. Foiled In Burkina Faso, the announcement in 2014 that long-serving president Blaise Compaore sought to extend his rule beyond 30 years brought hundreds of thousands of protesters onto the streets. Compaore was forced to stand down. In Malawi, the parliament in 2002 blocked Bakili Muluzi from seeking a third mandate in 2004. Zambia's then president, Frederick Chiluba, tried in vain to change the constitution in 2001 in order to get a third term, but was forced to bow to popular pressure.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.