Those look like citrus mites to me. But it also looks like your tree may have been suffering from heat and/or moisture stress. This makes it much more susceptible to infestations of this sort, so as you treat this you need to check your cultivation practices and correct anything that is amiss.
There are chemical treatments for citrus mites. You can also try insecticidal soap spray following a thorough warm spray bath with a significant amount of water being directed under the leaves to wash the pests away. The aim is to knock them off the underside of the leaves. You can also wipe the leaves after spraying with a paper towel to try to remove even more of the pests. Unfortunately, this will likely take several treatment sessions, a few days apart, to make a good dent in their population. And, as mentioned, if your tree remains stressed, it will likely attract pests again in short order.