Salesforce REST API without sharing

Salesforce REST API without sharing

I'm getting this error:
Implementation restriction: activity aggregate relationships only allow
security evaluation for non-admin users when a single parent record is
evaluated
When I do this query:
https://na1.salesforce.com/services/data/v28.0/query?q=SELECT Id, (SELECT
Id, Subject, ActivityType, Description FROM ActivityHistories) FROM
Opportunity
After reading this post, I think I need to change public with sharing
class to public without sharing, in APEX. How to achieve this if I'm
issuing REST API through POST request directly?