<apex:page controller="GetLeads">
  <apex:form >
      <apex:pageBlock >
          <apex:pageBlockSection title="Select Campaign">
              <apex:pageblockSectionItem >
                  <apex:outputLabel value="Campaigns" />
                    <apex:selectList size="1" value="{!selectedCampaign}">
                        <apex:selectOptions value="{!campaignLst}" />
                        <apex:actionSupport event="onchange" action="{!fetchLeads}" rerender="leads"/>
                    </apex:selectList>
              </apex:pageblockSectionItem>
          </apex:pageBlockSection>
          <apex:outputPanel id="leads">
              <apex:outputPanel rendered="{!displayLeads}">
              <apex:pageBlockSection title="Leads of Selected Campaign" >
                  <apex:pageBlockTable value="{!leadLst}" var="lead" rendered="{!leadLst.size > 0}">
                      <apex:column headerValue="Name" value="{!lead.Name}"/>
                  </apex:pageBlockTable>
                  <apex:outputText value="No Leads for selected Campaign" rendered="{!NOT(leadLst.size > 0)}" />
                </apex:pageBlockSection>    
              </apex:outputPanel>
          </apex:outputPanel>
      </apex:pageBlock>
  </apex:form>
</apex:page>

public class GetLeads {
    public List<SelectOption> campaignLst{get;set;}
    public String selectedCampaign{get;set;}
    public List<Lead> leadLst{get;set;}
    public Boolean displayLeads{get;set;}
    
    public GetLeads(){
        campaignLst = new List<SelectOption>();
        leadLst =new List<Lead>();
        campaignLst.add(new SelectOption('none','-none-'));
        for(Campaign camp : [SELECT Id,Name FROM Campaign Order By Name ASC]){
           campaignLst.add(new SelectOption(camp.Id,camp.Name));             
        }
    }
    public PageReference fetchLeads(){
        leadLst.clear();
        displayLeads = false;
        if(selectedCampaign != 'none'){
            displayLeads = true;
            Set<Id> leadIds = new Set<Id>();
            for(CampaignMember campMem : [SELECT Id, LeadId FROM CampaignMember WHERE CampaignId =:selectedCampaign AND LeadId != NULL]){
                if(campMem.LeadId != null ){
                    leadIds.add(campMem.LeadId);        
                }
            }
            if(!leadIds.isEmpty()){
                leadLst = [SELECT Id,Name FROM Lead WHERE Id IN :leadIds];        
            }  
        }   
        return null;
    }
}