It wasn't a very happy 27th birthday for singer Juelz Santana: Sources say his blowout bash on Monday night was ruined by Snoop Dogg and several other rowdy partygoers.

An insider spotted Snoop arriving at the fete at M2 Ultralounge with an entourage of more than 20 and a four-man security team, which clogged W. 29th St. so badly that the NYPD came to investigate.

"Snoop's caravan was beyond outrageous," says the spy. "It was taking up the entire block."

The scene was so disastrous that police closed the street before many guests had even arrived.

Among the friends of Santana unable to get through the mess was none other than Diddy, who was to host the evening.

"When Diddy saw how chaotic it was, he refused to get out of the car," the insider says. "He said he'd come back later, but never did."

That's probably a good thing. More chaos ensued inside the club when a partygoer knocked a bottle off a VIP table, sending glass flying and triggering a major brawl.

"There was a fight that took over much of the VIP area, and because it was so crowded, it took a little while to break up," the source says, adding that both Santana (who sings the hit "There It Go") and rapper Maino were near the altercation but didn't get involved. "It was total madness."

The only person who DID have a good time? The oblivious Snoop, who held court at his own table and was seen dancing with Lil' Kim while security removed the brawlers.

But he did make it up to his buddy Santana by the end of the night. An M2 insider says Snoop bought $8,000 worth of Champagne for the singer's table before leaving with his entourage.