Other than assertions to that effect by the Trump Administration, there is no evidence of this. However there is very strong evidence that it did not happen.
There are a fair amount of pictures of the outside of the BOK Center during the time in question. They pretty much all show just a small crowd.
As a bit of background, I'm a Tulsan who has attended multiple events at the BOK in the past. In every instance, the metal detectors were just inside the main doors. However, it appears that for this event they were expecting a large enough crowd that a cordon of barricades was erected around the area, with the entrance controlled by metal detectors and a temperature taking station.
Here are a couple of pictures of that station, taken by Tulsa World reporter Samantha Vicent, and posted on Twitter:
This one pretty clearly shows there wasn't really any line at all for the metal detectors.
Here's another that she confirmed was taken from outside the cordon looking toward the detector station (she was not allowed in due to carrying a backpack). There is no line or protestors visible for quite a distance.
Here's a cap of some on-the-ground reporting from David Weigel of the Washington Post stating that protestors were remarkable mostly by their absence, and protestors were in fact being kept very far away:
Here's another picture and story showing that a couple arriving late had no trouble whatsoever getting in.
Weigel also posted a video here showing the outdoor "overflow" stage, and a smallish glob of red-hatted folks lined up to enter (the big glass building is the BOK). Here's another showing people entering, with nowhere near the kind of line the routing barriers seemed to be anticipating.
So there was no big backup of frustrated prospective attendees hoping to get in. Multiple journalists working the event reported that latecomers had no trouble whatsoever walking up and getting in. There is also no sign whatsoever of any protestors or anyone being disruptive in any way.
Another good thing to look at would be arrest records at the event. It turns out there was one protestor arrested for obstructing the rally. One. Here's her story:
Buck put on her black “I Can’t Breathe” shirt and walked to the
barricade near the BOK, where she said security let her through. She
said she didn’t make it far, though, as Trump campaign folks quickly
“I was surrounded and was told I could not go any further and that I
was trespassing on a private event and I wasn’t wanted there,” Buck
said. “And I said, ‘This is my town; this is my city. I’m a citizen of
the United States of America.’
“And I was just praying as people were looking at me. And I realized
they were going to arrest me.”
Buck said she kept reiterating that she had a ticket but that Trump’s
campaign people wanted her to walk out. She said she sat down on the
ground because she wasn’t going to walk away.
To be clear, Buck had made it in past the metal detectors, and was arrested for "obstruction" (which seemed to involve sitting down) inside the cordon.
Given the fact that there was only one arrest, and how little action it took on her part to get surrounded and arrested, any claim that there were in fact large numbers of other protestors who made it as far as the entrance metal detectors and were not surrounded and arrested, or reported on, seems incredibly unlikely.
Finally, the main person making this claim in the first place, Brad Parscale, has now completely changed his story, and as of this writing is claiming it was actually the Tulsa Police who kept Trump supporters away. Whatever the merits of that claim may be, it is pretty clearly a different and incompatible claim.
In an interview, Mr. Parscale said the empty arena was not his fault,
and that local law enforcement in Tulsa had overreacted, making it
difficult for supporters to gain entry. He claimed to have thousands
of emails from supporters who tried to get into the Bank of Oklahoma
Center and were turned away, but he did not share those messages or
names of supporters.