MANILA - Some 400,000 people clad in all shades of pink gathered on Saturday (April 23) for what was both a campaign and a birthday rally for Vice-President Leni Robredo in a show of force meant to demonstrate that she is very much in the running to become the next president of the Philippines.

Her supporters, who began assembling at noon, filled a sun-baked, 2-km stretch of an eight-lane highway just south of the capital, Manila.

Some had walked for 4km to reach the venue. Tens of thousands more were sheltering inside malls, waiting for the searing heat to let up.

One mall owned by the family of Mr Jaime Zobel de Ayala, one of the country's wealthiest men, provided shuttle services to the rally site.

Stations were set up where those on-site could have special placards made, or get an ice cream and pink-coloured salt bread.

"I want to be here. I want to be represented," said Ms Marivic Asis, 19, a nursing student.

By early evening, the rally site was even more packed. Dozens of huge screens were set up on mini-stages throughout the venue for everyone to see what was happening on the main stage.

Among the crowd were volunteers doling out bottled water. Many who had already found a spot to settle in were fanning those who were still snaking through the crowd.

As of 8pm, though, Ms Robredo had yet to make an appearance.

Earlier in the day, she clinched an endorsement from former Islamist rebels who now govern an autonomous Muslim-majority region in the southern island of Mindanao.

"This is a very special birthday gift," she told top officials of the Bangsamoro Autonomous Region in Muslim Mindanao.

About 1,000 Catholic bishops and priests and one of the country's largest labour organisations also announced their support for Ms Robredo on her birthday.

Ms Robredo, 57, still trails Mr Ferdinand Marcos Jr, the 64-year-old son and namesake of the late dictator, by some 30 percentage points in opinion polls.

But her supporters insist there is a disconnect between survey results and Ms Robredo's massive campaign rallies.

Saturday's rally surpassed her biggest so far, held in a province north of Manila that drew some 200,000.

Mr Marcos and his running mate, Ms Sara Duterte-Carpio, 43, daughter of President Rodrigo Duterte, staged their own sizable rally in Manila's heartland, just 5km away from Ms Robredo's.

"The victory party is here," said Ms Duterte-Carpio

Police estimated that some 12,000 were at that rally.

Many received green plastic fans with Ms Duterte-Carpio's name on them, but no one was giving away free bottled water.

On May 9, over 65 million Filipinos will cast their ballots to choose who will succeed President Duterte.

More than 18,000 posts are also up for grabs in the national election, from president all the way down to town councillor.

Ms Robredo narrowly defeated Mr Marcos Jr in the 2016 vice-presidential race.

But his campaign for the nation's top office has been boosted by Ms Duterte-Carpio and a massive misinformation effort on social media.

• With additional reporting by Dana Batnag