The island nation has been gripped by constitutional turmoil since President Maithripala Sirisena sacked his former ally, prime minister Ranil Wickremesinghe, on October 26 and appointed Rajapakse in his place.
Busloads of Rajapakse followers were set to arrive from across the country for the rally planned near parliament in the capital Colombo.
Parliament has been suspended while Rajapakse seeks support for a vote of confidence by tempting defectors from other parties.
Tens of thousands attended a rally in support of Wickremesinghe last week, amid warnings from the parliamentary speaker Karu Jayasuriya that the dispute could end in a "bloodbath" if MPs are barred from holding a vote.
In his most strongly worded statement to date, the speaker warned Monday he would not recognise Rajapakse as prime minister and that the president had violated the constitution by sacking Wickremesinghe and suspending parliament.
"Until the new faction (of Rajapakse) is able to prove a majority in parliament, I will recognise the situation that prevailed prior to October 26," Jayasuriya said.
Sirisena announced on Sunday that parliament would reconvene on November 14, a week later than he had promised, prolonging the power struggle that has crippled the country.
The speaker said Sirisena had gone back on previous commitments to recall parliament this week.
"I have to agree with the majority of parliament who believe that the president's actions are undemocratic, unconstitutional and against all norms of parliamentary procedure," Jayasuriya said.
While president from 2005 to 2015, Rajapakse ended a 37-year-old Tamil separatist war but has been accused of human rights violations, nepotism and corruption.
Sri Lankan activists hold placards during a demonstration in Colombo yesterday.
Thousands of supporters of former Sri Lankan president Mahinda Rajapakse headed for the capital on Monday to rally in support of his controversial nomination as prime minister, as a political crisis deepened.
