president Alexander Lukashenko

Vladimir Putin has offered to help ensure Belarus’s security, according to its president Alexander Lukashenko, as pressure builds on the strongman leader and opposition protesters prepare for a show of force Sunday.

Thousands demonstrated in the capital Minsk Saturday after main election challenger Svetlana Tikhanovskaya asked supporters to rally over the weekend and keep alive a movement that poses the biggest challenge to Lukashenko’s hold over the ex-Soviet country.

Many gathered at the spot where Alexander Taraikovsky, 34, died on Monday during protests against an election the opposition says was rigged to give Lukashenko another term in office.

Demonstrators heaped …