President Vladimir Putin has ruled Russia since the turn of the century, transforming its tumultuous post-Soviet democracy into a highly-repressive authoritarian regime.

As the ex-KGB agent looks set to clinch an unprecedented fifth term in office at elections this March, here are five key moments that helped define his time in the Kremlin.

- The Kursk tragedy -

In August 2000 -- just four months after Putin was first elected president -- the Kursk nuclear submarine sank in the Arctic, trapping all 118 crew on board.

Putin drew ire for failing to interrupt his holiday on the Black Sea, and when he appeared in public four days after the incident, he claimed the crisis was under control.

When authorities finally met families of the victims, video footage showed the grieving mother of one crew member shouting in frustration before being drugged with sedatives against her will.

Moscow eventually accepted foreign help to retrieve the submarine a week after the accident, but by then the entire crew had died, in what was a huge early blow to Putin's image.

- Beslan school bloodbath -

On September 1, 2004, at the beginning of the school year in Russia, Islamic extremists stormed a school in the town of Beslan in the North Caucasus.

Hundreds of students, parents and teachers were held for more than 50 hours in an ordeal that ended in a bloodbath and the deaths of over 330 people -- including 186 children -- after security forces stormed the school.

The Kremlin was criticised for its handling of the crisis, especially after an Islamist hostage-taking at a theatre in Moscow two years earlier ended in some 170 deaths, mostly civilians killed by gas used by security services.

Putin's ruthless war against Islamist insurgents in Chechnya in 1999 had helped fuel his initial popularity, but at the end of 2019 he described Beslan as a "personal pain" that would remain with him for life.

- President swap -

When Putin reached the end of his presidential term limit in 2008, he temporarily handed the Kremlin to Dmitry Medvedev in what critics described as a tactical move designed to keep the long-term leader in power.

Demonstrations and clashes with police erupted in central Moscow in late 2011 to protest the results of parliamentary elections and Putin's announcement that he would run for the presidency again.

The huge protests were harshly put down and many who took part were handed long jail terms.

Putin began his second stint in the Kremlin in 2012 with a crackdown on the opposition, branding some NGOs and media outlets "foreign agents" and reinforcing conservative values with a law prohibiting "homosexual propaganda".

The Kremlin chief cemented his long-stay in power by reforming the constitution in 2020, allowing him to remain in post until 2036.

- Attacks on Ukraine -

In 2014, Russia hosted the Sochi Winter Games -- the most expensive Olympics yet -- in a project that won the Kremlin short-lived international praise.

Just one month later, riding high on an upsurge in popularity after the games and emboldened by political turmoil in neighbouring Ukraine, Putin ordered Russian troops into Crimea.

His swift and audacious annexation of the peninsula set off a chain reaction that culminated in armed separatists launching a rebellion in the country's east.

The conflict sparked condemnation and sanctions from the West, but did little to deter the Kremlin leader from ordering a devastating military assault on the whole of Ukraine on February 24, 2022.

- Wagner mutiny -

In June 2023, Yevgeny Prigozhin -- leader of the Wagner mercenary group that spearheaded some of the Kremlin's bloodiest battles in Ukraine -- led a short-lived mutiny against Moscow's military leadership.

He had for months accused the Russian defence ministry of stifling his forces' progress on the backfield by withholding crucial ammunition.

While Wagner forces eventually turned back after a hastily-organised amnesty deal with Belarus, Putin described the rebellion as a "stab in the back".

Exactly two months after the failed mutiny, Prigozhin was killed in a mysterious plane crash.

The Kremlin denied it was responsible, but the crash nevertheless sent a chilling message to Putin's critics.