Billionaire heir Anant Ambani and fiancée Radhika Merchant's lavish pre-wedding celebrations continue with FOUR-DAY Mediterranean cruise - that include private performances from Katy Perry and the Backstreet Boys

Billionaire Anant Ambani and fiancée Radhika Merchant's insanely lavish pre-wedding celebrations have continued - as the couple embarked on a four-day Mediterranean cruise with 1,200 guests, throwing parties across Europe with private performances from stars like Katy Perry, Pitbull, and the Backstreet Boys.

Anant, 29, who is the son of India's richest man, Mukesh Ambani, and Radhika, 29, are set to tie the knot in Mumbai in July, but the festivities kicked off months ago. 

Back in March, the couple made headlines after it was revealed that they had hosted a wildly glamorous party to celebrate their upcoming nuptials in Jamnagar, India, that was estimated to have cost them more than $128 million.

Now, the pre-wedding events have resumed, as the couple recently took more than 1,000 of their friends and family members - including global tech CEOs, Bollywood stars, and Hollywood legends - on a private cruise through the Mediterranean.

Over the course of the four-day expedition, the soon-to-be husband and wife, along with their guests, stopped in stunning European cities like Sicily and Rome in Italy and Cannes in France.

Billionaire Anant Ambani and Radhika Merchant's lavish pre-wedding celebrations have continued - as the couple embarked on a four-day Mediterranean cruise with 1,200 guests

Anant, 29, who is the son of India's richest man, Mukesh Ambani, and Radhika (seen last year), 29, are set to tie the knot in Mumbai in July, but the festivities have already kicked off

Each night of the trip, the couple hosted exquisite parties that had opulent décor and star-studded celebrity performances. Katy Perry is seen performing

Each night of the trip they hosted exquisite parties that had opulent décor and star-studded celebrity performances.

'It was a retreat for the people that have contributed to our life in different ways,' Radhika told Vogue recently of the cruise.

The publication reported that they decided to host the endeavor because 'many of their close friends' were 'unable to make it to' the first party, and that they ultimately decided on a cruise through Europe because they 'struggled to find a destination that could accommodate such a large group.'

The vacation began in Palermo, Sicily, on May 29, where guests enjoyed a welcome lunch before heading onto the boat.

An onlooker uploaded footage that showed dozens of elite black cars lined up by the pier dropping the guests off.

The vessel they stayed on is called the Celebrity Ascent, and is said to have cost roughly $900 million to build.

It's unclear how much it costs to rent out the entire thing like Anant and Radhika did, but the cheapest cruise is priced at $849 for one person for seven nights.

The ship boasts a whopping 32 restaurants, bars, and lounges, as well as a rooftop garden, a casino, spa, multiple pools, and a full theater.

The group stopped in stunning European cities like Sicily, Rome, Cannes. Bollywood stars Shanaya Kapoor, Ananya Panday, and Suhana Khan are seen in Italy during the trip

The vacation began in Palermo, Sicily, on May 29. An onlooker uploaded footage that showed dozens of elite black cars lined up by the pier dropping the guests off

The vessel they stayed on is called the Celebrity Ascent, and is said to have cost roughly $900 million to build

The ship boasts a whopping 32 restaurants, bars, and lounges, as well as a rooftop garden, a casino, spa, multiple pools, and a full theater

On the first night, the couple hosted a 'starry-night-themed formal party,' according to Vogue.

On the first night, the couple hosted a 'starry-night-themed formal party,' according to Vogue, and the Backstreet Boys hit the stage to perform during the star-studded event

For dazzling soiree, Radhika opted for a custom black and white Robert Wun gown that had a love letter Anant wrote to her years prior sewn into the skirt.

'He wrote me this long letter for my [22nd] birthday about what I mean to him,' Radhika recalled.

'I wanted [to save] it for posterity - I want to be able to show it to my kids and grandkids, and say that "this is what our love was."'

The Backstreet Boys also hit the stage to perform during the star-studded event.

The next day, the group arrived in Rome, where the guests were free to sightsee, before heading to a second bash that evening - a toga party on the ship's deck.

For this one, Radhika donned another custom look, this time created by Grace Ling.

It contained 'an aerospace aluminum breastplate, made to mimic drapery, that the designer 3D-carved, sculpted, and then 3D-printed,' Vogue reported.

French DJ David Guetta entertained the crowd throughout the night.

The next day, the group arrived in Rome, where the guests went sightseeing, before heading to a second bash that evening - a toga party on the ship's deck. Guests are seen in Rome

On the third day, there was another party - a masquerade ball at the acclaimed Château de la Croix des Gardes in Cannes. Radhika is seen at the bash

For this celebration, Radhika wore a custom Versace dress which contained a diamond-encrusted chest piece and a long flowing blue skirt. She's seen at the party with her parents

They spent the third day in Cannes, where the bride and group held another party for the guests - a masquerade ball at the acclaimed Château de la Croix des Gardes, the palace famously featured in Alfred Hitchcock's movie To Catch a Thief. 

For this celebration, Radhika wore a custom Versace dress that took over a year to complete.

It contained a diamond-encrusted chest piece and a long flowing blue skirt. 

She paired the gown with a stunning blue opal necklace that sat on a diamond chain - her and Anant's birthstones - which was made by Lorraine Schwartz. 

Katy Perry sang during that party, and as she belted out her hit Firework, an intricate firework show blasted across the Cannes sky.

After getting back on the boat, the festivities continued with an afterparty hosted by Pitbull.

The trip ended in Portofino, where they turned the main square into an 'open-air market complete with food stalls.' 

They enlisted acclaimed event planner Vincenzo Dascanio to make sure it was perfect, and he did not disappoint.

She paired the gown with a stunning blue opal necklace that sat on a diamond chain - her and Anant's birthstones - which was made by Lorraine Schwartz

Katy sang during that party, and as she belted out her hit Firework, an intricate firework show blasted across the Cannes sky

The trip ended in Portofino, where they turned the main square into an 'open-air market complete with food stalls.' More guests are seen in Italy during the trip

Celebrity attendees on the cruise included Bollywood stars Ananya, Suhana (seen), and Shanaya, as well actor Adam Sandler

According to Vogue, there will be 'several more high-profile happenings to come.' Suhana and Shanaya are seen in Portofino during the cruise

Italian singer Andrea Bocelli performed, singing the classic tune Can't Help Falling in Love for the crowd, which Radhika described as 'magical.'

For the final party, she opted for a hot pink archival Yves Saint Laurent for Dior dress, which was sourced by stylist Mimi Cuttrell.

'It seems like a simple dress, but if you look at the seams and the construction, you'll see how detailed it really is,' she gushed. 

Other celebrity attendees on the cruise include actor Adam Sandler and Bollywood stars Ananya Panday, Suhana Khan, Shanaya Kapoor, Sara Ali Khan, Manushi Chhillar, Shah Rukh Khan, and Guru Randhawa.

According to Vogue, there will be 'several more high-profile happenings to come,' including 'multiple pujas (a prayer ritual for good luck) and 'a traditional sangeet ceremony.'

Anant's father Mukesh, 67, runs Indian oil-to-telecoms giant Reliance Industries, and is worth more than $114 billion.

Radhika also hails from a well-off family, as her dad is the CEO of Encore Healthcare.