(Reuters) – Libya’s Muammar Gaddafi is just the latest name to be struck from the list of Africa’s so-called "Big Men”, the dwindling band of strongarm rulers who are finding it ever harder to keep their grip on power.

But while the death of Gaddafi was at least partly the result of outside intervention, it is domestic pressure from Africa’s increasingly demanding city-dwellers that is forcing the pace of wider political change on the continent.

Recent African elections, while in many cases either flawed or tilted towards the incumbent, have shown they can replace the coup d’etat as the preferred way of ousting leaders who have outstayed their welcome.

"It is the ballot box not the bullet that is effecting political change in Africa,” said Tara O’Connor of London-based Africa Risk Consulting.

"The pressure for this is local and sustained.”

Killed by NATO-backed fighters who overran his home town Sirte on Thursday , Gaddafi topped the list of Africa’s longest serving rulers with 42 years of autocratic and often bloody and eccentric power to his name.

That put him ahead of President Teodoro Obiang Nguema Mbasogo of tiny central African oil-producer Equatorial Guinea on 32 years, Angola’s Jose Eduardo dos Santos on the same, and Zimbabwe’s Robert Mugabe on 31 years.

The Two-Decade-Plus Club already lost two members this year with the "Arab Spring” uprisings that saw off Tunisia’s Zine al-Abidine Ben Ali and Egypt’s Hosni Mubarak.

CONTROLLING SIX KEY INSTITUTIONS

South of the Sahara, the push for change has been less spectacular, and some may doubt whether it is happening at all.

Burkina Faso’s Blaise Compaore extended his 24-year rule in April with a landslide 80 percent poll win. Preliminary results from Cameroon’s election this month showed Paul Biya due to add to his 29 years in power with a 77 percent score.

Both votes suffered from a weak opposition, allegations of irregularities and an apathetic electorate convinced its vote would change nothing — the classic combination favouring the incumbent with a firm grip on the levers of state machinery.

Ghanaian democracy activist George Ayitteh said authoritarian rulers kept going by wielding influence over six vital institutions: the security forces, media, civil service, parliament, judiciary and election commission.

But of those six, Ayitteh said the media was increasingly slipping out of the reach of such leaders because of the growth of privately-owned FM radio stations and an expansion of the Internet, particularly in the cities.

"It is almost impossible for them to control the Internet, while radio is a very powerful medium in Africa. It is going to be harder for them to stay in power,” said Ayitteh, who runs the Washington-based Free Africa Foundation.

The 1989 fall of the Berlin Wall and collapse of the Soviet Union brought to an end Africa’s role as a proxy for Cold War rivalries, with superpowers propping up ideologically compliant dictators. A shift in French policy also means the incumbent in its ex-colonies no longer enjoys automatic support from Paris.

Ivory Coast’s Laurent Gbagbo learnt that to his cost in April when, after having allowed his security forces to fire live rounds at street protesters, he was removed from office by rebel forces backed by French troops.

Yet the most powerful factor of all may be the relentless demographic trend which is turning Africa into a continent dominated by young urbanites.

PRESSURE FROM THE CITIES

While many of the remaining Big Men are well into their 70s, over half Africa’s billion-plus population is under 25 and in many cases have known nothing but city life, said O’Connor.

"Urbanisation is breaking down those traditional ethnic allegiances. The common factor now is demands on jobs, health and housing — that puts pressure on the politicians.”

That pressure would be all the greater were it not for the fragmented opposition in many African states, unable to put internecine rivalries aside to form a united front. Cameroon’s Biya, for example, stood against over 20 opponents.

Both O’Connor and Attiyeh acknowledged it could take time for viable opposition forces to develop in some countries, but noted that, unlike two decades ago, all-out boycotts of polls handing the incumbent a walkover were now rare.

Relatively smooth elections in countries from Nigeria and Zambia, which last month led to the swearing-in of opposition leader Michael Sata, are offering Africans proof they can throw out leaders deemed not to be delivering.

Attiyeh said the Arab Spring uprisings, while not emulated further south, had created a sense of impatience for change among many Africans which in coming years may lead to a series of clashes between them and the remaining old-time leaders.

"It is not always going to be smooth sailing,” he warned.