Nicki Minaj's show won't go on in Romania ... 'cause she just pulled the plug on her concert in Bucharest -- saying her security detail tells her it's not safe.

The singer-songwriter announced the news less than an hour ago on her X account ... letting her millions of followers know she won't hit the stage in Eastern Europe -- 'cause her team says protests in the area could explode into violence.

NM says she's a mom first ... and, she needs to think about her safety for her son and the other team members traveling with her -- so, she just can't take the risk.

Minaj tells her fans she loves them and hopes they'll understand her decision ... before adding she's heading back to London to perform a show later this week.

Romanian activists plan to march on Bucharest's town center tomorrow morning -- about 12 hours from now -- to protest the state's fiscal policies. Unclear why Nicki's team thinks violence may explode ... but, clearly she's not comfortable taking the risk.

BTW ... mixed reactions on social media -- with some fans sending Nicki love while others claim there's a very small amount of protesting in the area, so Minaj just doesn't really want to come to Bucharest.

As you know, not the first time Nicki's had to cancel a show last minute ... remember, in late May, Nicki got arrested at the airport in Amsterdam -- and the delay caused her to miss her show in Manchester that night.

Anyhoo ... like Nicki says, the "Pink Friday 2" tour rolls on in London next week -- and, Romanian Barbz will just have to wait until the next time Nicki comes to town.