The flash point of petrol is high also the compression ratio, spark plug used for Petrol engine, valves used for injection of fuel in diesel, is it OK 

we cannot use petrol in diesel engines or vice versa because diesel and petrol has different characteristics (flashpoint) every engine has unique design depends on its fuel to be use. petrol and diesel engine has different engine stroke cycle specially during intake and ignition stroke

Diesel

intake stroke: enters hot air

ignition during power stroke: sprays pressurised diesel fuel on the combustion chamber by means of injection nozzle 

gasoline/petrol

intake stroke: enters air and fuel mixtures by means of carburator

ignition during power stroke: ignites by means of sparkplug sparks inside combustion chamber @ highly compressed air and fuel mixtures

Compression Ratios are also very different, Diesel run with compression ratios from 17~21 to 1 where gasoline engines can only run at compression Ratios of 8~11 to 1.  Gasoline will detonate at much lower compression than what is needed for diesel to reach self ignition point.
A real diesel engine will be more efficient overall since it will have the ability to operate at higher compression ratios. If a petrol engine were to have the same compression ratio, then knocking (self-ignition) would occur and this would severely reduce the efficiency, whereas in a diesel engine, the self ignition is the desired behavior.

You’re all right, but there are multi fuel outboard engines (based on petrol engine design) used in defence running on various fuels, kerosene, jet fuel or diesel (low compression ratio with spark plug support). Downside is that the engines are not compliant to enviro regulations, due to incomplete combustion (engines are “Smokey”).
See Mercury Racing Optimax JP and Evinrude E-TEC. 

Gasoline, or petrol, ignites too soon (predetonation) in a diesel engine due to diesel engines high compression ratios.  That causes some reverse loading, very high stresses, mostly on the cylinder rods which may eventually fail in high compression load and bearing wear.

Multifuel engines must be designed for extra high rod loads.

Many of the comments here are salient, but there is one area that has not been mentioned....the inadvertent use of petrol in a diesel engine.

I am fortunate enough never to have done so, but I know many who have used the wrong pump at the filling-station. What few realise is if you drive a diesel and realise after a few seconds of filling the error that has been made, it is possible to just fill the rest with diesel without any real problems. it will run a little rough for that tank, but will revert to normal when next refilled.

It IS critical though that it is only a little petrol/gasoline though...if it is a lot - say greater than 10% - it would be a drain-down for safety, but less than that should not cause any great issue. 

Whilst I have never tried it nor know any that have (other than a complete refuel....doh!), due to the need to physically ignite the cylinder charge, I very much doubt that this would be the same for diesel in a petrol engine...