An Air Carrier Captain reported they refused ATC clearance for takeoff because they observed another aircraft at the opposite end of the runway facing them.

Narrative

After lengthy delay with traffic saturation taking off and landing on Runway XX. Operations also occurring on Runway XY; we were cleared onto Runway XX for line up and wait. We taxied into position. Tower then cleared crossing traffic from Runway XY towards terminal while we waited in position. There were multiple aircraft including Runway XY and what appeared to be Taxiway 1. Then Tower cleared us to takeoff once traffic at opposite end was cleared. There appeared to be an aircraft facing us at opposite end with lights off when he gave us this clearance to takeoff. We emphatically rejected the clearance until all aircraft were off the Runway. Opposing aircraft was at far end but Tower seemed so task saturated that we did not trust his calls. This aircraft appeared to have come from Taxiway 2 onto Runway XZ facing us and just sat there. When we rejected takeoff clearance; aircraft seemed to make a left off around Taxiway 3 or 1. Once aircraft were all cleared; we were then given the appropriate clearance. We took off with no incident. This is my second ASAP in less than 2 weeks at this same Airport. This Airport is so task saturated that it is an accident waiting to happen. First Officer said the same thing as I was saying it. There are numerous aircraft taking off and landing on two Runways (normally XX and XY) with small aircraft taking off at midfield. Hard to get a word in on the comms because they are way too busy. ATC at this Airport seems overwhelmed with work load. They may need to consider a flow/metering into and out of this Airport. This Airport needs to find a better way to operate especially around the holidays when traffic saturation is high. They need better procedures and better operational risk management. They may need to consider a flow/metering into and out of this Airport. Understand this is a foreign/international Airport with their own set of country rules but a future discussion should be considered for safety reasons.